Earth systems and living things

Where the answer comes from: the passage says hydro, wind and solar "are replaced naturally and will not run out on a human time scale." Those three are the renewable sources. Coal and natural gas are fossil fuels, and uranium for nuclear power "is not replaced by nature."

Step by step:

1. List the renewable sources: hydro, wind, solar.
2. Read their 2023 (green) bars: hydro 8%, wind 17%, solar 8%.
3. The largest is wind at 17%.

Natural gas (34%) is the largest bar overall, but it is a fossil fuel, so it does not answer the question.

The answer is wind.

Tip: When a question adds a condition such as renewable, filter the data by that condition first, then compare. The biggest bar on the graph is often a trap.

Where the answer comes from: the table row for 2018 reads "| 2018 | 85 | 88 | 41,000 |".

Step by step:

1. Find the column "Days with no rain, June–September" and look down it for 88.
2. The value 88 appears in the 2018 row.
3. Move right along the same row to "Acres burned": 41,000.
4. Watch out for 2020 and 2022: they show 88 in a different column (average summer high temperature, °F), not dry days.

The answer is 41,000 acres.

Check: No other year has 88 in the dry-days column (the others are 61, 70, 79, 65 and 92).

Tip: When a number appears in more than one column, say the column name to yourself before you read across.

Where the answer comes from: the Saffir-Simpson table in paragraph 2: Category 2 is "96–110" mph and Category 4 is "130–156" mph.

Step by step:

1. 105 mph is between 96 and 110, so the storm began as Category 2.
2. 135 mph is above the Category 3 range (111–129) and between 130 and 156, so it became Category 4.
3. The storm skipped right past Category 3 during those 12 hours.

The hurricane went from Category 2 to Category 4.

Tip: With range tables, check both ends of the range. A value just past the top of one row (135 > 129) belongs in the next row.

Where the answer comes from: paragraph 3 defines storm surge as "a rise in sea level pushed ashore by the storm's winds." Paragraph 4 says "Raising homes on stilts or pilings keeps living space above floodwater, and restoring coastal marshes and barrier islands helps absorb some of the surge."

Step by step:

1. Identify the hazard: storm surge is flooding by seawater.
2. Sort the actions in paragraph 4: roof straps and shutters protect against wind; raised homes and restored marshes protect against water.
3. Only one choice combines the water-related actions.

The best plan is to raise homes on pilings and restore the marshes along the shore.

Tip: Match the solution to the specific hazard named in the question. A true statement about a different hazard (wind) is still a wrong answer.

Where the answer comes from: the graph. Nitrate falls from 9.0 mg/L at Site A to 1.0 mg/L at Site E. Dissolved oxygen rises from 3.2 mg/L at Site A to 8.4 mg/L at Site E.

Step by step:

1. Trace the red (nitrate) line: 9.0 → 7.5 → 5.0 → 2.5 → 1.0. It decreases downstream.
2. Trace the blue (dissolved oxygen) line: 3.2 → 4.0 → 5.6 → 7.1 → 8.4. It increases downstream.
3. The two move in opposite directions: where nitrate is high, oxygen is low. This is an inverse relationship, and it fits the background: extra nutrients feed algae, and decomposing algae use up oxygen.

The answer is as nitrate decreases downstream, dissolved oxygen increases.

Tip: On a two-line graph, use the legend to keep the lines straight, and describe each line separately before comparing them.

Where the answer comes from: the passage says all five sites are "downstream of the fields." So every site might receive runoff, and there is no site without it to compare against.

Step by step:

1. The suspected cause is fertilizer runoff from the fields.
2. A good test compares water that gets runoff with similar water that does not. A site upstream of the fields is the same stream but without the farm runoff, so it acts as a control.
3. If the upstream site has low nitrate and high oxygen, while Site A has high nitrate and low oxygen, the fields are the likely cause. If the upstream site also has low oxygen, something else is going on.
4. The other choices repeat data already collected or measure something that cannot separate the causes.

The best choice is measuring nitrate and dissolved oxygen at a site upstream of the fields.

Tip: On investigation-design questions, look for the choice that adds a control group or removes another possible cause.

Where the answer comes from: Graph 2 gives depth to the water table: 20 m (1980), 21 m (1990), 27 m (2000), 36 m (2010), 43 m (2020). The passage says "A larger depth means the water level is lower."

Step by step:

1. Find each ten-year change in depth:
   • 1980–1990: 21 - 20 = 1 m
   • 1990–2000: 27 - 21 = 6 m
   • 2000–2010: 36 - 27 = 9 m
   • 2010–2020: 43 - 36 = 7 m
2. The largest change is 9 m, between 2000 and 2010.
3. This matches Graph 1: withdrawal peaked at 3.4 km³ in 2010, the farthest above recharge.

The water table dropped most between 2000 and 2010.

Tip: "Dropped the most" asks about change. Subtract neighboring values instead of picking the largest number.

Where the answer comes from: the table pairs each lake's pH with its fish species: 4.6 → 0, 5.0 → 1, 5.4 → 3, 6.4 → 6, 6.8 → 7, 7.1 → 8.

Step by step:

1. The lakes are already listed from lowest to highest pH.
2. As pH goes up, the number of fish species goes up every time: 0, 1, 3, 6, 7, 8.
3. A higher pH means less acidic water. So less acidic lakes have more fish species, and more acidic lakes have fewer.

The answer is as lake pH increases, the number of fish species increases.

Check: The most acidic lake (North, pH 4.6) has no fish, and the least acidic lake (Silver, pH 7.1) has the most (8).

Tip: On the pH scale, lower numbers mean more acidic. Say to yourself low pH = strong acid before you describe a trend.

Where the answer comes from: the passage says "All plots had the same soil and the same 4% slope, and all received the same natural rainfall for one year." It also says "each plot got a different winter treatment."

Step by step:

1. Independent variable (changed on purpose): the winter treatment (plowing or not, cover crop or not).
2. Dependent variable (measured): the soil lost.
3. Controlled variables (kept the same): soil type, slope and rainfall.
4. Only one answer choice is a controlled variable: the slope of the land.

The answer is the slope of the land.

Check: If one plot were steeper, water would run off it faster, and that plot might lose more soil for a reason unrelated to its treatment.

Tip: Sort every variable into three bins: changed, measured, kept the same. The controlled variables are the ones that make the test fair.

Where the answer comes from: in the table, visits rise and fall with the AQI (AQI 45 → 14 visits; AQI 160 → 38 visits; AQI 70 → 20 visits). But the high temperature also rose and fell the same week (82 °F up to 99 °F and back to 88 °F), and the data cover only one week.

Step by step:

1. The data do show a strong pattern: the highest AQI day (Friday) has the most visits.
2. A pattern like this is a correlation. It is consistent with the claim but does not prove it.
3. Something else changed on the same days. Heat can also trigger breathing problems, so heat could explain part of the rise.
4. One week is a very small sample. More weeks, or a comparison of hot days with low AQI, would be needed.

The best evaluation: the data support the claim but do not prove it, because temperature rose and fell along with the AQI and only one week was studied.

Tip: On the GED, "prove" is a red-flag word. A correlation supports a claim; it does not prove cause.

Where the answer comes from: the passage explains that the algae "make food by photosynthesis and pass much of it to the coral." During bleaching, corals push out the algae, and "without the food from the algae they starve and catch diseases more easily." It adds that "long or repeated heat waves kill them."

Step by step:

1. Bleaching is the loss of the algae, not death itself. The passage says "Bleached corals are not dead yet."
2. The algae supply much of the coral's food.
3. If the heat lasts, the algae stay away, so the coral goes without food for a long time and starves or gets sick.

The answer is the corals lose the algae that provide much of their food, so they starve and become more likely to get diseases.

Check: This also explains why corals can recover if the water cools within a few weeks: the algae return before the coral starves.

Tip: For "why" questions, trace the chain the passage gives (heat → algae leave → no food → starvation), and pick the choice that names the key link.

Where the answer comes from: Graph 1 shows recharge at "2.0 every year" from 1980 to 2020, while withdrawal rose from 1.5 to as high as 3.4 km³ per year.

Step by step:

1. The claim says less refilling (recharge) caused the drop. If that were true, the recharge line would fall.
2. Instead, recharge stayed flat at 2.0 km³ per year. What changed was pumping: withdrawal climbed above recharge after the 1980s.
3. So the evidence points to overpumping, not drought, as the main cause.
4. The falling water table (choice B) is real, but it fits both explanations, so it cannot challenge the claim.

The best evidence is that recharge stayed at 2.0 km³ per year while withdrawal rose above it.

Tip: To challenge a cause-and-effect claim, find data showing the supposed cause did not change, or that something else did.

Where the answer comes from: the passage defines the 100-year floodplain as land with "a 1% chance of flooding in any given year" and warns: "The chance is the same every year, so two large floods can happen in back-to-back years."

Step by step:

1. A "100-year flood" describes a probability, not a schedule.
2. Each year, land in the 100-year floodplain has a 1% chance of a flood that big.
3. A flood in 2023 does not use up the chance. In 2024 the chance is still 1%.
4. So the resident is wrong: a flood that size could happen any year, even the next one.

The best response is the resident is mistaken; the chance of a flood that size is still about 1% every year, including 2024.

Check: Over many centuries, a 1% yearly chance averages about one such flood per 100 years, which is where the name comes from. The average is not a promise.

Tip: Probabilities of independent events do not "reset" after the event happens. Watch for the gambler's mistake on hazard questions.

Where the answer comes from: the passage claims that "limestone (calcium carbonate) neutralizes some of the acid." But the limestone lakes might also differ in other ways (size, depth, forests, location), so comparing the lakes alone cannot prove the cause.

Step by step:

1. To test one cause, change only that cause and keep everything else the same.
2. Split water from one acidic lake into identical containers. This makes the water, the amount and the temperature the same.
3. Add crushed limestone to some containers (experimental group) and nothing to others (control group).
4. Measure pH in both. If only the limestone containers rise in pH, limestone is the cause.

The best experiment is to divide water from North Lake into identical containers, add crushed limestone to half, add nothing to the other half, and compare the pH.

Tip: "Best test" questions usually want a controlled experiment: one change, a control group, everything else equal.

Where the answer comes from: the table shows that bleaching rises as the heat rises and lasts longer: 0.5 °C for 2 weeks → 5%; 1.0 °C for 4 weeks → 22%; 1.5 °C for 6 weeks → 48%; 2.0 °C for 8 weeks → 71%.

Step by step:

1. The worst year in the table is 2020: 2.0 °C above for 8 weeks, with 71% bleached.
2. The forecast (2.5 °C for 10 weeks) is both hotter and longer than 2020.
3. Following the trend, bleaching should be greater than 71%.
4. The passage adds that "long or repeated heat waves kill them," so many bleached corals may not recover.

The most likely outcome is more than 71% of the coral bleaches.

Check: 2022 (1.2 °C, 3 weeks, 30%) fits the same trend: it lies between 2017 and 2019 in both heat and bleaching.

Tip: When you predict beyond the data, first find the most extreme data point, then decide which side of it the new case falls on.

Where the answer comes from: the AQI row (45, 62, 98, 135, 160, 120, 70), the visits row (14, 15, 21, 30, 38, 33, 20) and the category table ("| 151–200 | Unhealthy |").

Step by step:

1. Days with an AQI above 100: Thursday (135), Friday (160) and Saturday (120). Their visits were 30, 38 and 33, all at least 30. ✓
2. Unhealthy means 151–200. Only Friday (160) is in that range, so three days is false. Thursday and Saturday are Unhealthy for sensitive groups.
3. The most visits (38) were on Friday, not Saturday.
4. The most visits were on the hottest day (99 °F), not the coolest.

The supported statement is on every day with an AQI above 100, there were at least 30 asthma visits.

Tip: For which is true questions, test each choice against the data and cross out any choice that fails even one check.

Where the answer comes from: Table 2 says making aluminum from recycled scrap saves "about 95%" of the energy needed to make it from ore.

Step by step:

1. Say making all the aluminum from ore uses 100 units of energy.
2. After the switch, 60% still comes from ore: 0.60 \times 100 = 60 units.
3. 40% comes from scrap, which uses only 5% of the usual energy: 0.40 \times 100 \times 0.05 = 2 units.
4. New total: 60 + 2 = 62 units, down from 100.
5. Energy saved: 100 - 62 = 38, a 38% drop. (Shortcut: 0.40 \times 95\% = 38\%.)

Total energy use falls by about 38%.

Check: The answer must be less than 40% (the most possible if recycling saved all its energy) and less than 95% (only part of the metal is recycled) ✓.

Tip: When only part of a total changes, multiply the part's share by the saving. Do not just add or pick one of the given percents.

Where the answer comes from: the table rows "| EF4 | 166–200 | Well-built houses leveled |" and "| EF5 | over 200 | Strong-frame houses swept off their foundations |".

Step by step:

1. Worst damage: well-built houses leveled. That matches EF4.
2. No house was swept away, so the tornado did not reach EF5.
3. The EF4 range is 166–200 mph. The lowest estimate in that range is 166 mph.

The lowest estimated wind speed is 166 mph.

Check: 165 mph would still be EF3, where entire stories are destroyed but houses are not leveled ✓.

Tip: In work-backward questions, find the row first, then read the end of the range the question asks for (lowest = start, highest = end).

Where the answer comes from: the passage says a wildfire needs "fuel (dry grass, brush and dead trees), oxygen, and heat." It describes prescribed burns as fires set on purpose "to remove fuel that has built up on the forest floor."

Step by step:

1. The problem is too much fuel: decades of dead trees and brush.
2. A fire's intensity depends heavily on how much fuel it can burn.
3. A prescribed burn removes that fuel in safe weather, before a wildfire can use it.
4. The other plans either add fuel, help only after a fire starts, or target a cause that is not the main problem.

The best plan is to carry out prescribed burns in safe weather to remove the built-up fuel.

Tip: For mitigation questions, name the exact problem in the stem first (here: built-up fuel). Then choose the plan that attacks that problem directly.

Where the answer comes from: the passage states that coral reefs "provide food and shelter for roughly 25% of all ocean species."

Step by step:

1. Living corals build and maintain the reef structure.
2. Many fish depend on the reef for food (animals and algae living on the reef) and for shelter from predators.
3. When corals die, that food and shelter shrink, so fish that depend on the reef lose habitat.
4. Their numbers are likely to fall, or they must move elsewhere.

The most likely effect is the number of fish that depend on the reef for food and shelter will decrease.

Check: This matches a general rule in ecology: when a habitat-building species declines, the species that rely on it decline too.

Tip: For effect on organisms questions, look for what the changed part of the system provides (food, shelter, oxygen), then follow it to the organisms that need it.

Where the answer comes from: the passage says ores "form over millions of years, so they are nonrenewable," and that "Recycling does not create new metal, but it lets the same metal be used again, so less new ore must be mined."

Step by step:

1. Aluminum is made from bauxite ore, which takes millions of years to form.
2. A resource is renewable only if nature replaces it on a human time scale. Nature does not replace mined bauxite that fast.
3. Recycling reuses aluminum that was already mined. It slows how fast bauxite is used up, and it saves about 95% of the energy, but it does not create new ore.
4. So the student mixes up recyclable with renewable.

The best response is aluminum is still nonrenewable, because its ore forms over millions of years; recycling only makes the existing supply last longer.

Tip: Renewable = nature makes more quickly. Recyclable = people can reuse what they have. A resource can be recyclable and still nonrenewable.

Where the answer comes from: the graph (coal 45% and natural gas 25% in 2013; coal 18% and natural gas 34% in 2023) and the table (coal 1.0 kg, natural gas 0.4 kg, others about 0 kg).

Step by step:

1. Picture 100 kWh of electricity, so each percent equals 1 kWh.
2. 2013: 45(1.0) + 25(0.4) = 45 + 10 = 55 \text{ kg CO}_2
3. 2023: 18(1.0) + 34(0.4) = 18 + 13.6 = 31.6 \text{ kg CO}_2
4. Percent decrease: \frac{55 - 31.6}{55} \times 100\% = \frac{23.4}{55} \times 100\% \approx 42.5\%
5. Natural gas rose, but it releases far less CO₂ than the coal it replaced, and wind and solar added almost none.

Lakeland released about 43% less CO₂ per kWh in 2023.

Tip: With two sources, write down what each one gives you before calculating. Percentage points on a graph (coal fell 27 points) are not the same as the percent change in the final result.

Where the answer comes from: the table shows Saturday with 120 thousand vehicles (about 90,000 fewer than a weekday) but an AQI of 120. On Monday, Tuesday and Wednesday, with about 210,000 vehicles, the AQI was only 45, 62 and 98.

Step by step:

1. If weekday traffic were the main cause, the AQI should be worst on high-traffic weekdays and better on low-traffic weekend days.
2. Saturday had much less traffic than Monday–Wednesday, yet its AQI (120) was higher than all three.
3. Saturday was also very hot (97 °F). The passage says ozone "builds up fastest on hot, sunny days," so heat may matter more than the number of cars.
4. The other choices either support the claim or do not test it.

The best challenge is Saturday had about 90,000 fewer vehicles than a weekday, yet its AQI was higher than on Monday, Tuesday or Wednesday.

Tip: To challenge a cause, find a case where the supposed cause is weak but the effect is still strong, or the other way around.

Where the answer comes from: in the original study, "All plots had the same soil and the same 4% slope, and all received the same natural rainfall." The second study changes both the cover crop and the slope at the same time.

Step by step:

1. Water runs off a steep hillside faster and carries more soil than on flat ground.
2. Farm A had a cover crop (less erosion) but a steep slope (more erosion). These two effects can cancel out.
3. Because slope was not controlled, similar soil loss does not show that cover crops fail.
4. A fair test would compare fields with the same slope, soil and rainfall, changing only the cover crop.

The main flaw is that the fields differed in slope as well as in cover crop, so slope could explain the result.

Tip: When a study changes two things at once, you cannot tell which one caused the result. Look for the uncontrolled variable.

Where the answer comes from: the table rows "| 2020 | 88 | 79 | 36,000 |" and "| 2022 | 88 | 65 | 15,500 |".

Step by step:

1. If temperature alone controlled acres burned, two years with the same temperature should burn about the same area.
2. 2020 and 2022 both averaged 88 °F, yet 2022 burned 15,500 acres, less than half of 2020's 36,000.
3. The two years differ in dry days (79 vs 65). Fewer dry days in 2022 lines up with fewer acres, which points to dryness as another important factor.
4. The other choices show temperature and acres rising together. They support the claim rather than challenge it.

The best challenge is 2020 and 2022 had the same average temperature (88 °F), but 2022 burned less than half as many acres and had 14 fewer dry days.

Tip: "Alone" claims are challenged by a case where the named cause stays the same but the effect changes.