Integrated Science Review Lesson Test (40 Questions)

Answer: C

Correct answer: Plant height after three weeks. The dependent variable is what you measure to see the effect, while the independent variable is what you deliberately change. A helpful test: the dependent variable depends on the independent one. Here fertilizer amount is changed on purpose, so it is independent, and the height that results is dependent. Factors kept the same, like seed variety, are controlled variables, and tools such as a ruler are measuring instruments, not variables in the experiment.

• A: Ruler length is a measuring tool property, not the response.
• B: Seed variety is a potential controlled variable.
• C (correct): Plant height is the measured response.
• D: Fertilizer is deliberately varied, so it is independent.

Answer: D

Correct answer: Nucleus. In eukaryotic cells such as human cells, the nucleus is enclosed by a double membrane and holds the chromosomes, which carry nearly all of the cell's genes. Mitochondria keep a small circular DNA of their own, but it is only a tiny fraction of the total. Ribosomes are a common trap because they work closely with genetic information, but they only read messenger RNA copies to build proteins; they do not store the DNA.

• A: The membrane controls material exchange; it is not the main DNA store.
• B: Ribosomes make proteins using genetic instructions.
• C: Golgi packages and sorts cellular products.
• D (correct): Most of a human cell's DNA is housed in the nucleus; only a small amount is in mitochondria.

Answer: D

Correct answer: Newton. Newton's second law, F = ma, defines the unit: one newton is the force that accelerates a 1-kilogram mass at 1 meter per second squared, so 1 N = 1 kg·m/s². The joule, watt and kelvin are easy to mix up because they also appear in physics problems, but they measure energy, power and temperature. A useful memory link is that the unit for force carries the name of the scientist whose laws describe force and motion.

• A: Kelvin measures thermodynamic temperature.
• B: A joule measures energy.
• C: A watt measures power.
• D (correct): A newton (N) is the SI unit for the net force that can accelerate mass, such as the force in F = ma.

Answer: B

Correct answer: Condensation. When moist air rises, it expands and cools. Cooler air can hold less water vapor, so the vapor condenses onto tiny particles of dust or salt, forming the droplets that make up a cloud. The same process makes water bead up on a cold glass. Sublimation is easy to confuse with condensation, but it changes a solid directly into a gas, so it produces vapor rather than liquid droplets. The reverse of condensation is evaporation.

• A: Erosion removes and transports rock or soil particles.
• B (correct): Condensation converts vapor to liquid droplets.
• C: Sublimation converts solid directly to gas.
• D: Infiltration is water entering soil or rock.

Answer: B

Correct answer: Energy in food moving from rabbit to fox. In a food chain, each arrow points from the organism being eaten to the one that eats it, showing the direction energy and nutrients travel. Read the arrow as "is eaten by": grass is eaten by the rabbit, which is eaten by the fox. A common error is to read the arrow as "eats," which reverses the meaning. Energy flows one way through the chain and is not recycled, while matter can be recycled by decomposers.

• A: Energy flows toward the eater, not in reverse here.
• B (correct): The fox consumes the rabbit, so energy moves to the fox.
• C: The food-chain arrow is not a DNA-inheritance symbol.
• D: Sunlight is not recycled through the chain.

Answer: A

Correct answer: Number of neutrons. The atomic number, 6 protons, identifies carbon, so both isotopes share it. The numbers 12 and 14 are mass numbers, and the difference comes from carbon-14 having 2 more neutrons.

• A (correct): Isotopes of one element have the same protons but different neutron counts, which changes their mass.
• B: Every carbon atom has 6 protons; changing the proton count would make a different element.
• C: A neutral atom has as many electrons as protons, so both neutral isotopes have 6 electrons.
• D: The atomic number is the proton count, which is 6 for all carbon atoms.

Answer: C

Correct answer: 0 N. Newton's first law says an object keeps moving at a constant velocity unless a net force acts on it. Constant velocity means no change in speed or direction, so acceleration is zero, and by F = ma the net force is zero regardless of mass. Forces may still act, such as gravity and friction, but they balance. The usual mistake is thinking that motion requires a continuing net force; a net force is needed only to change motion.

• A: Weight can be balanced by other forces.
• B: 100 N has no basis without a stated acceleration.
• C (correct): Zero acceleration requires zero net force.
• D: Mass in kilograms is not force in newtons.

Answer: A

Correct answer: It converts light energy into stored chemical energy. In chloroplasts, pigments such as chlorophyll absorb light, and that energy is used to build glucose from carbon dioxide and water, releasing oxygen. The energy is then stored in the chemical bonds of glucose. No matter is created; the atoms already present in carbon dioxide and water are rearranged. Plants also carry out cellular respiration all the time, breaking down that glucose to release energy for their cells.

• A (correct): Photosynthesis captures light energy and stores it in the chemical bonds of sugars.
• B: Atoms are rearranged; matter is not created from nothing.
• C: Plants carry out both photosynthesis and cellular respiration.
• D: Photosynthesis occurs in plants, algae and some bacteria, not animal muscle.

Answer: C

Correct answer: Earth’s rotation on its axis. Earth spins once about every 24 hours, carrying each location into sunlight and then into darkness, which produces day and night. Revolution, Earth's yearlong trip around the Sun, works with axial tilt to create seasons, not daily cycles. A handy way to keep them straight: rotation rhymes with spinning in place, while revolution is going around. Earth's distance from the Sun changes only slightly and slowly over a year, so it cannot explain a daily cycle.

• A: Revolution is involved in annual seasons but not the direct daily cycle.
• B: Lunar shadows cause limited solar eclipses, not each night.
• C (correct): As Earth rotates, locations face toward or away from the Sun.
• D: Distance changes do not make a location face away every day.

Answer: A

Correct answer: It changes physically to solid water. Freezing is a physical change: water molecules slow down and lock into a crystal pattern, but each molecule is still H₂O. Physical changes alter state, shape or size without forming new substances, and they are often easy to reverse, as when ice melts. A chemical change, like burning or rusting, forms new substances. Ice still contains thermal energy too; its molecules keep vibrating in place, and only at absolute zero would that motion reach its minimum.

• A (correct): A phase change retains the chemical identity H₂O.
• B: Freezing does not create new elements.
• C: Solid water still has internal energy.
• D: Proton counts stay unchanged.

Answer: D

Correct answer: 3 g/mL. Density is mass divided by volume. Measuring the water displaced by an irregular object, such as a rock, gives its volume, since the rock pushes aside exactly its own volume of water. Here 48 g divided by 16 mL is 3 g/mL. To avoid reversing the ratio, remember the units: grams per milliliter means grams on top. A density of 3 g/mL is typical for many rocks and is greater than water's 1 g/mL, which is why the rock sinks.

• A: 64 adds mass and volume, which have incompatible units.
• B: Dividing volume by mass reverses the density ratio.
• C: 32 comes from incorrect subtraction rather than division.
• D (correct): Density = 48 g / 16 mL = 3 g/mL.

Answer: C

Correct answer: Microbial respiration during decomposition. Extra nutrients such as nitrogen and phosphorus fuel a rapid algal bloom. When the algae die, bacteria and other decomposers break down the large mass of dead material and use dissolved oxygen in their respiration, which can leave too little oxygen for fish. This chain of events is called eutrophication. A common mix-up is assuming the living algae deplete the oxygen directly; during daylight they actually add oxygen through photosynthesis, and the big drop comes from decomposition.

• A: Oxygen atoms are not converted into light as claimed.
• B: Seasonal sunlight is not the direct decomposer mechanism described.
• C (correct): Microorganisms can use dissolved oxygen while breaking down biomass.
• D: Gravity does not disappear under these conditions.

Answer: C

Correct answer: Mass 60 kg; weight about 98 N. Mass stays 60 kg everywhere. Weight on Earth is

and on the Moon it is about one sixth of that.

• A: 588 N is her weight on Earth; the Moon's weaker gravity cuts it to about one sixth.
• B: This reverses the ideas: mass stays at 60 kg, while weight is what drops on the Moon.
• C (correct): Mass does not depend on location, and her weight is 60 × 9.8 ÷ 6, or about 98 N.
• D: The weight is right, but mass is the amount of matter and does not shrink on the Moon.

Answer: D

Correct answer: 3 m/s². Newton's second law, F = ma, rearranges to

Dividing the 15 N net force by the 5 kg mass gives 3 m/s². A unit check confirms it: a newton is a kg·m/s², so newtons divided by kilograms leaves m/s², the unit for acceleration. Multiplying force by mass or dividing mass by force produces units that are not acceleration, which signals an error. Larger mass means less acceleration for the same force.

• A: Adding force and mass is dimensionally invalid.
• B: This incorrectly divides mass by force.
• C: Multiplication gives the wrong rearrangement of F = ma.
• D (correct): Acceleration = $\frac{15}{5}$ = 3 $\frac{\mathrm{m}}{{\mathrm{s}}^2}$.

Answer: B

Correct answer: Comparing randomized plant groups treated alike except for fertilizer dose. A fair test changes only one variable, the independent variable, and keeps everything else the same, so any difference in growth can be traced to that variable. Random assignment prevents hidden differences, such as healthier seedlings, from piling up in one group, and using groups instead of single plants reduces chance effects. Changing light and water at the same time makes it impossible to tell which factor caused the result.

• A: Changing light and water together prevents isolating any effect of fertilizer.
• B (correct): Holding other factors equal and varying only fertilizer directly tests the hypothesis.
• C: Two uncontrolled examples do not establish cause.
• D: Marketing claims are not controlled measurements.

Answer: C

Correct answer: 50%. Set up a Punnett square: the Aa parent can pass on A or a, each half the time, while the aa parent can pass on only a. The four boxes give Aa, Aa, aa, aa, so half of the offspring are expected to be aa. This cross is called a test cross, because crossing an unknown individual with a recessive one reveals its hidden alleles. The familiar 25% figure applies only when both parents are Aa.

• A: Some offspring can inherit the dominant A allele.
• B: An aa combination is possible from $\frac{A}{a}$ and a parents.
• C (correct): Half the gametes from Aa contain a, and aa contributes a: $\frac{1}{2}$ aa.
• D: 25% would match Aa × Aa, not this cross.

Answer: B

Correct answer: 3 m. The wave equation is speed = frequency × wavelength, so wavelength = speed ÷ frequency. A frequency of 100 Hz means 100 waves pass each second, and the sound travels 300 m in that second, so each wave must be

m long. A unit check helps: m/s divided by 1/s leaves meters. Higher frequency at the same speed means a shorter wavelength, which is why high-pitched sounds have short waves.

• A: This multiplies instead of divides.
• B (correct): Wavelength = speed / frequency = $\frac{300}{100}$ = 3 m.
• C: This is the reversed calculation.
• D: This adds unlike quantities.

Answer: C

Correct answer: They shrink as water moves out of them. In osmosis, water moves across a membrane toward the side with more dissolved solute. The salty water outside is hypertonic to the cells, so water leaves them and they shrink.

• A: Cell membranes let water pass, which is exactly why osmosis can occur.
• B: Water would move in only if the outside had less dissolved solute than the inside, a hypotonic solution.
• C (correct): The outside solution is hypertonic, so water diffuses across the membrane toward the saltier side, out of the cells.
• D: Osmosis is the movement of water toward the higher solute concentration, so water leaves rather than enters.

Answer: A

Correct answer: Precise but not accurate. Precision is how closely repeated readings agree, and accuracy is how close they are to the true value. These readings differ by only 0.01 g, so they are precise, but every one is about 2.3 g too high, so the balance is not accurate. Averaging more readings will not remove this consistent offset; the balance needs calibration.

• A (correct): The readings agree closely with one another, but all sit about 2.3 g above the true 50.00 g value.
• B: The readings are far from the true value yet very close together, which is the reverse of this description.
• C: Close agreement among readings shows precision, but a consistent 2.3 g offset means the balance is not accurate.
• D: Readings that differ by only 0.01 g are highly precise, even though they miss the true value.

Answer: B

Correct answer: The pool has more thermal energy, but the cocoa has higher average particle kinetic energy. Temperature measures the average kinetic energy of particles, so the 70 °C cocoa is hotter. Thermal energy is the total energy of all the particles, so the far larger mass of pool water holds much more of it.

• A: Higher temperature means higher average particle energy, but thermal energy also depends on the amount of material, so the small mug cannot match the pool's total.
• B (correct): Temperature tracks average particle energy, which is higher in the hot cocoa, but the pool's huge mass gives it more total thermal energy.
• C: Temperature measures average particle energy, not total energy, and 26 °C is lower than 70 °C.
• D: Thermal energy depends on both temperature and mass, and these differ greatly.

Answer: D

Correct answer: 0 m/s. Velocity is a vector: it depends on displacement, the straight-line change in position from start to finish, including direction. Walking 30 m east and then 30 m west returns the student to the starting point, so the displacement is zero and the average velocity is zero. Average speed is different; it uses total distance, here 60 m over 30 s, or 2 m/s. Confusing speed with velocity is the main trap in problems like this.

• A: The route ends at its starting position.
• B: 2 $\frac{\mathrm{m}}{\mathrm{s}}$ is average speed, not net displacement divided by time.
• C: 60 m is a distance, not a velocity.
• D (correct): Net displacement is zero, so average velocity is zero.

Answer: D

Correct answer: Weathering breaks down rock in place; erosion moves the material away. Weathering can be mechanical, such as frost wedging when water freezes and expands in cracks, or chemical, such as acid rain dissolving limestone. Erosion then carries the loosened pieces away by water, wind, ice or gravity, and deposition drops them somewhere else. A simple memory hook: weathering breaks, erosion takes. The two often work together, which is why they are confused, but the key difference is whether the material moves.

• A: Wind, water and ice can erode surface material.
• B: Evaporation is a water phase change, not the breakdown or movement of rock.
• C: The definitions are reversed, and erosion does not always create rock.
• D (correct): Weathering breaks down rock where it sits; erosion removes and carries the pieces elsewhere.

Answer: C

Correct answer: Its velocity direction is continually changing. Velocity includes both speed and direction, so acceleration is any change in either one. A satellite in a circular orbit keeps a steady speed, but its direction changes at every moment as it curves around Earth. That change is centripetal acceleration, pointed toward Earth's center and supplied by gravity. The common misconception is that acceleration only means speeding up; slowing down and turning are also accelerations, which is why a car's steering wheel counts as an accelerator.

• A: Turning at fixed speed still changes velocity.
• B: Gravity is what bends the satellite’s path.
• C (correct): Acceleration measures change in velocity, including direction.
• D: No change in satellite mass is required.

Answer: B

Correct answer: The surviving population may be enriched for heritable resistant variants. Bacterial populations contain natural genetic variation, and a few cells may already carry resistance genes before treatment. The antibiotic kills susceptible cells, while resistant ones survive and divide, passing resistance to their offspring. Over time, resistant variants make up more of the population. This is natural selection acting on existing variation. The tempting error is thinking the drug causes each bacterium to develop resistance on purpose; mutations arise randomly, not because they are needed.

• A: Selection can change genetic composition of populations.
• B (correct): Resistant variants can survive and reproduce, increasing in frequency.
• C: Evolution does not require goal-directed mutation in each organism.
• D: Antibiotics do not transmute elements.

Answer: D

Correct answer: Town A, with 1.5 percent versus 0.3 percent. Compare rates, not totals. For Town A,

or 1.5 percent. For Town B,

or 0.3 percent. Town A's rate is five times higher even though it had fewer cases.

• A: The town is right, but the percentages are off by a factor of 10 from a decimal slip.
• B: This compares raw counts and ignores that Town B has 20 times as many residents.
• C: 6 percent comes from dividing Town B's cases by Town A's population, mixing up the denominators.
• D (correct): 30 ÷ 2,000 is 1.5 percent and 120 ÷ 40,000 is 0.3 percent, so Town A's rate is higher.

Answer: D

Correct answer: Changing Earth-Moon-Sun viewing geometry as the Moon orbits. The Sun always lights half of the Moon. As the Moon orbits Earth over about 29.5 days, we see different portions of that lit half: none at new moon, half of it at the quarter phases, and all of it at full moon. The most common misconception is that Earth's shadow causes phases, but that happens only during lunar eclipses, which are rare and occur at full moon.

• A: Earth’s shadow is relevant to lunar eclipses, not regular phases.
• B: The Moon reflects sunlight rather than producing visible light itself.
• C: The Moon stays gravitationally associated with Earth.
• D (correct): Viewing geometry determines how much of the illuminated half is visible.

Answer: B

Correct answer: A convergent boundary, with earthquakes, volcanoes and possible tsunamis. When an oceanic plate sinks beneath another plate, the plates are moving together, which is a convergent boundary called a subduction zone. These zones produce volcanoes, deep trenches, strong earthquakes and possible tsunamis.

• A: At divergent boundaries plates separate, as at mid-ocean ridges; here one plate is sinking under another.
• B (correct): Plates colliding with one sinking below the other is a subduction zone, which builds volcanoes and produces large earthquakes and tsunamis.
• C: Transform boundaries have plates sliding sideways, not one plate diving beneath the other.
• D: Colliding plates store and release great strain, so subduction zones are among the most earthquake-prone places on Earth.

Answer: A

Correct answer: Carbon in much new plant biomass comes from atmospheric CO₂. During photosynthesis, plants take in carbon dioxide through tiny leaf pores called stomata and use light energy to build it into sugars, which become cellulose and other parts of wood. Soil supplies water and minerals such as nitrogen, but only a small share of a tree's dry mass. It surprises many people that most of a tree's solid mass essentially came out of the air. Sunlight supplies only the energy for the process.

• A (correct): Photosynthetic carbon fixation incorporates carbon from CO₂.
• B: Soil supplies water and minerals but not all structural carbon.
• C: Photons supply energy, not carbon atoms.
• D: Plant respiration and photosynthesis can occur in the same organism.

Answer: A

Correct answer: 5 mg/L. Concentration is the amount of solute divided by the volume it is spread through. Here 15 mg spread evenly through 3 L gives

mg in each liter. The unit itself shows the operation: mg/L means milligrams divided by liters. Multiplying or adding the values gives numbers that do not describe how much solute is in each liter. Water-quality limits for substances such as nitrate are usually reported in exactly these units.

• A (correct): 15 mg / 3 L = 5 $\frac{\mathrm{mg}}{\mathrm{L}}$.
• B: This is the reciprocal of the stated ratio with inappropriate units.
• C: Multiplication gives total scale, not concentration.
• D: Addition of milligrams and liters is not meaningful.

Answer: D

Correct answer: Sediment washed in by rain can also reduce clarity. A causal claim is weakened by a plausible alternative explanation for the same result. Cloudiness, or turbidity, can come from algae, but also from suspended soil and silt carried in by runoff. If rain recently stirred up sediment, the cloudiness could have nothing to do with algae. To settle the question, you would examine a sample, for example by checking for green color, chlorophyll, or particles that settle out over time.

• A: Being alive does not distinguish algae from sediment as the cause.
• B: Natural water can be cloudy for several reasons.
• C: Solar distance is unrelated to identifying pond turbidity.
• D (correct): Suspended soil is a plausible competing cause of turbidity.

Answer: A

Correct answer: 0.7 g/cm³; it floats. First find the volume, then divide mass by volume.

Since 0.7 g/cm³ is less than water's 1 g/cm³, the block floats.

• A (correct): The volume is 100 cm³, so the density is 70 ÷ 100 = 0.7 g/cm³, which is less than water's.
• B: This uses a volume of 10 cm³, but 8 × 5 × 2.5 is actually 100 cm³.
• C: This divides volume by mass instead of mass by volume; the units would come out as cm³ per gram.
• D: The density is right, but an object less dense than water floats rather than sinks.

Answer: A

Correct answer: It rises sixteenfold from 10 to 160 mg/s. The load, or mass carried per second, equals concentration times flow, since mg/L multiplied by L/s gives mg/s. Before the rain,

mg/s; after it,

mg/s. When two multiplied factors each grow fourfold, the product grows

times. The common mistake is to track only one change, either concentration or flow, which gives a fourfold increase.

• A (correct): Load = concentration × flow: 2×5=10 $\frac{\mathrm{mg}}{\mathrm{s}}$, 8×20=160 $\frac{\mathrm{mg}}{\mathrm{s}}$; factor 16.
• B: Fourfold counts only one of the two changes.
• C: Both concentration and flow change.
• D: This considers neither full concentration nor flow ratio.

Answer: C

Correct answer: 70 percent; the other 150 J became mostly thermal energy. Efficiency is useful output divided by total input:

or 70 percent. Energy is conserved, so the remaining

J was not lost from existence; it became less useful forms, mostly heat.

• A: 30 percent is the share wasted, not the efficiency, and the wasted amount is 150 J, not 350 J.
• B: This divides input by output; no real device can deliver more energy than it takes in.
• C (correct): 350 ÷ 500 = 0.70, and by the first law the missing 150 J changed form, mostly into heat.
• D: Energy is never destroyed; the 150 J still exists, mostly as thermal energy in the motor and air.

Answer: B

Correct answer: 50 J. Each transfer passes along only about 10% of the energy, because most of the rest is used for life processes and lost as heat. Multiply by 0.1 once per step: 50,000 J becomes 5,000 J, then 500 J, then 50 J after three transfers. Count the arrows carefully, because stopping one step early is the usual error. This steep loss explains why food chains rarely have more than four or five levels and why top predators are relatively few.

• A: 5,000 J corresponds to one transfer.
• B (correct): 50,000 × $0.1^3$ = 50 J.
• C: 500 J corresponds to two transfers.
• D: Energy transfers are not zero, and matter conservation does not imply no trophic transfer.

Answer: D

Correct answer: Temperature and oxygen are confounders that differ with treatment. A confounding variable changes along with the treatment and could itself cause the effect. Warmer water and more oxygen can both speed fish growth, so any gain in the test tanks cannot be credited to the nutrient alone. A sound design keeps temperature, oxygen and other conditions the same in every tank and changes only the nutrient. When reviewing any study, ask what else differed between the groups besides the treatment.

• A: Nutrients can be studied experimentally.
• B: No such universal control outcome is guaranteed.
• C: Growth can be measured with a defined protocol.
• D (correct): Treatment is mixed with other possible causal variables, limiting attribution.

Answer: B

Correct answer: 128 m. An echo is a round trip: the sound travels to the cliff and back, so the total path is speed × time =

The cliff is half of that distance, 128 m. Forgetting to halve the result is the most common error. The same principle underlies sonar on ships and echolocation in bats, which judge distance from the delay between sending a pulse and hearing it return.

• A: 64 m uses an unnecessary factor of four.
• B (correct): One-way distance = 320 × $\frac{0.8}{2}$ = 128 m.
• C: 256 m is the total round-trip sound path.
• D: 400 m divides instead of multiplies the travel values.

Answer: A

Correct answer: Resistant carriers survive and reproduce more under the treatment environment. The resistant allele was already present before spraying, so no new mutation was needed. Spraying kills more susceptible insects, while carriers of the resistant allele survive and leave more offspring that inherit it. Repeated over generations, this raises the allele's frequency, which is natural selection. The tempting misconception is that insects develop resistance because they need it; individuals cannot change their genes on demand, but populations change as survivors reproduce.

• A (correct): Differential reproductive success can increase a preexisting allele’s frequency.
• B: The pattern is consistent with differential survival linked to resistance.
• C: Mutations are not reliably goal-directed by the organism’s need.
• D: Genetic variation is part of the scenario.

Answer: B

Correct answer: Day-night comes from rotation; seasons come from tilt during revolution. Rotation, spinning on the axis, carries each location into and out of starlight, so it sets the 20-hour day. Revolution sets the 500-day year, and because the axis is tilted, each hemisphere leans toward the star for part of the orbit and away for the rest, changing sun angle and day length. Distance changes are a common but mistaken explanation, since seasons are opposite in the two hemispheres at the same time.

• A: Distance alone does not explain seasons on a tilted planet.
• B (correct): Rotation sets the day, while tilt combined with orbital motion sets the seasonal pattern.
• C: Rotation sets the stated rotation period, not revolution alone.
• D: Latitude and axial tilt change illumination, so seasons differ by location.

Answer: A

The data show low humidity was present each time a fire began, which suggests it may be a necessary condition. It is not sufficient: fuel and something to ignite it are also required, and many dry days likely had no fire. Proving cause would need controlled comparisons rather than a single pattern.

• A (correct): Low humidity being present each time makes it look necessary, not sufficient, and a pattern alone does not prove cause.
• B: The data list only days that had fires; they do not show that every dry day produced a fire.
• C: A correlation can suggest a cause worth testing; it simply cannot prove one by itself.
• D: A factor present whenever an effect appears is not proof of cause; other needed conditions and confounders may be involved.

Answer: C

Correct answer: Retest with light varied and fertilizer held fixed, using replicated plants. In the original setup, light and fertilizer increased together, so their effects are confounded and cannot be separated. To isolate light, change only light and keep fertilizer, water, soil and temperature constant. Using several plants at each light level, called replication, reduces the chance that one unusual plant drives the result. Adding more changing factors or reporting only the best plant makes the evidence weaker, not stronger.

• A: Selective reporting weakens the evidence.
• B: Adding more changing factors makes attribution to light even harder.
• C (correct): Holding fertilizer constant while varying light, with several plants per level, isolates the light effect.
• D: Both light and fertilizer changed, so the cause is not isolated.

Answer: C

Correct answer: About 1,000 times. The pH scale is logarithmic: each one-unit drop means about ten times more hydrogen-ion activity. The drop is

units, so the increase is

times.

• A: This treats the pH scale as linear and just uses the 3-unit difference.
• B: A factor of 100 matches a 2-unit change; this drop is 3 units.
• C (correct): Each pH unit is a tenfold change, and three units give 10 × 10 × 10 = 1,000.
• D: Dividing 8 by 5 treats pH values as simple amounts instead of steps on a logarithmic scale.

Answer: A

Correct answer: 8 m/s. Net force: 2,700 N − 900 N = 1,800 N. Acceleration:

Starting from rest, speed after 4 s is 2 m/s² × 4

• A (correct): Net force is 1,800 N, giving 2 m/s² of acceleration, and 2 m/s² for 4 s gives 8 m/s.
• B: This uses the full 2,700 N pull and ignores the 900 N of resistance.
• C: This adds the resistance to the pull instead of subtracting it.
• D: This is the acceleration in m/s², not the speed reached after 4 s.

Answer: A

Correct answer: 2. Count oxygen atoms on the product side: CO2 has 2 and 2H2O has 2, for 4 in all. Each O2 molecule supplies 2 oxygen atoms, so

molecules of O2 balance the equation. Carbon (1) and hydrogen (4) already match.

• A (correct): The right side has 2 oxygen atoms in CO2 plus 2 in the two water molecules, a total of 4, which is two O2 molecules.
• B: One O2 supplies only 2 oxygen atoms, but the products contain 4 oxygen atoms.
• C: This counts the 4 oxygen atoms needed but forgets that each O2 molecule carries 2 of them.
• D: Three O2 molecules would supply 6 oxygen atoms, more than the 4 found in the products.

Answer: B

Correct answer: 9/16. A Punnett square for Rr × Rr gives RR, Rr, Rr and rr, so 3 of 4 outcomes show round seeds.

Each offspring is independent, so multiply:

• A: This is the chance for one offspring; both offspring must be round, so the probabilities must be multiplied.
• B (correct): Each offspring has a 3/4 chance of round seeds, and independent events multiply: 3/4 × 3/4 = 9/16.
• C: This is the chance that both offspring are wrinkled, using 1/4 × 1/4.
• D: This treats round and wrinkled as equally likely, ignoring that three of four combinations show the dominant trait.

Answer: D

Correct answer: Carbon dioxide. A compound contains two or more elements chemically bonded in a fixed ratio. Carbon dioxide always has one carbon atom for every two oxygen atoms. Copper is an element, and air and sweetened tea are mixtures whose proportions can change.

• A: Air is a mixture of gases such as nitrogen and oxygen in proportions that can vary.
• B: Copper is an element made of only one kind of atom.
• C: Sweetened tea is a mixture; you can add more or less sugar.
• D (correct): Carbon dioxide is carbon and oxygen chemically bonded in a fixed ratio, which makes it a compound.

Answer: D

Correct answer: River B, at 4 mg/L compared with 2 mg/L for River A. Concentration is amount per volume, so divide the pollutant per second by the water per second.

For River A, 600 mg/s ÷ 300

mg/L. For River B, 200 mg/s ÷ 50

mg/L. River B carries less pollutant in total but its water is twice as concentrated.

• A: A larger total load can come from carrying more water; concentration needs the amount per liter.
• B: The concentrations work out to 2 mg/L and 4 mg/L, which are not equal.
• C: This divides flow by load for River B instead of load by flow.
• D (correct): Dividing each load by its flow gives 600/300 = 2 mg/L for A and 200/50 = 4 mg/L for B.

Answer: C

Correct answer: More direct sunlight makes tomatoes ripen sooner. An observation records what happened, a hypothesis offers a testable explanation, and a prediction states a specific result that should follow if the hypothesis is right. Saying that more sunlight speeds ripening explains the pattern and can be tested, so it is the hypothesis.

• A: This is an observation, a direct record of what happened.
• B: This is another directly recorded observation, not a testable explanation.
• C (correct): This proposes a testable explanation for what was seen, which is what a hypothesis is.
• D: This is a prediction that would follow from a hypothesis, naming a specific checkable result.

Answer: B

Correct answer: A gas formed by the reaction escaped from the open cup into the air. Mass is conserved in a chemical reaction, so a missing mass means something crossed the boundary. The bubbles are a gas produced by the reaction, and in an open cup that gas escapes. In a sealed container, the total mass would stay the same.

• A: Ordinary reactions only rearrange atoms; they do not destroy them.
• B (correct): The fizzing released carbon dioxide gas, and its mass left the system through the open top.
• C: Ordinary chemical reactions do not convert a measurable amount of mass into energy.
• D: A change in density does not change mass; the scale measures the total amount of matter present.

Answer: C

Correct answer: The ozone layer. The ozone layer is the atmosphere's ultraviolet shield. Greenhouse gases such as carbon dioxide, water vapor and methane are a separate topic: they absorb the infrared radiation given off by Earth's warmed surface.

• A: Carbon dioxide is a greenhouse gas that absorbs outgoing infrared radiation, not most incoming ultraviolet.
• B: Water vapor mainly absorbs infrared radiation and helps the greenhouse effect.
• C (correct): The ozone layer absorbs much of the incoming ultraviolet radiation from the Sun.
• D: Methane is a greenhouse gas that traps infrared, not the main ultraviolet shield.

Answer: A

Correct answer: Divergent. Plates that pull apart form a divergent boundary. Magma rises to fill the gap and builds new crust, which is what happens along mid-ocean ridges.

• A (correct): At a divergent boundary plates pull apart and new crust forms from rising magma.
• B: At a convergent boundary plates collide, and one may sink beneath the other.
• C: At a transform boundary plates grind past each other sideways without creating new crust.
• D: Subduction happens where one plate sinks under another, the opposite of plates moving apart.

Answer: D

Correct answer: 6 m/s. First convert distance to meters: 3.6 km = 3,600 m. Then divide by time:

Checking that the units come out as meters per second confirms the division was done the right way.

• A: This divides 3.6 by 600 without first converting kilometers to meters.
• B: This divides time by distance, which would give seconds per meter, not meters per second.
• C: This is ten times too large, a decimal slip when converting 3.6 km to meters.
• D (correct): Converting 3.6 km to 3,600 m and dividing by 600 s gives 6 m/s.