When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly....

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Paradox

MEDIUM

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Notes

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When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.

The more slowly water moves, the more conducive its temperature is to the growth of algae.

When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.

Australian rivers dry up completely for short intervals in periods of extreme drought.

Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.

Solution

Passage Analysis:

Text from Passage	Analysis
When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly.	What it says: Less rain causes river water levels to drop and water to move more slowly What it does: Sets up the basic cause-and-effect relationship between rainfall and river conditions What it is: Factual observation about river behavior Visualization: Normal rain → Normal water level + Normal flow speed vs. Low rain → Low water level + Slow flow speed
Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain.	What it says: Algae thrive in slow water, so less rain leads to more algae per unit of water What it does: Connects the previous river conditions to algae growth, creating a logical chain What it is: Author's explanation linking environmental conditions to biological outcomes Visualization: Little rain → Slow river flow → More algae growth (maybe 30-40% more algae per unit)
By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.	What it says: After extreme droughts, algae levels stay low despite the slow-moving water that should help them grow What it does: Introduces a contradiction to the pattern just established What it is: Contrasting observation that creates a puzzle Visualization: Extreme drought → Very slow river flow → Low algae levels (opposite of what we'd expect from the previous pattern)

Argument Flow:

The passage sets up a logical pattern (less rain → slower rivers → more algae), then presents a contradiction (extreme drought → very slow rivers → but low algae). This creates a puzzle that needs explanation.

Main Conclusion:

There is no explicit conclusion in this passage - it's presenting a paradox that needs to be resolved by the answer choices.

Logical Structure:

This is a 'paradox' or 'discrepancy' structure where we establish a normal pattern, then show how extreme conditions break that pattern. The question asks us to explain why the extreme case doesn't follow the established rule.

Prethinking:

Question type:

Paradox - We need to explain why extreme drought leads to low algae levels despite creating the slow-moving water conditions that algae normally love

Precision of Claims

The claims involve specific conditions: 'less rainfall than normal' vs 'extreme drought', and algae levels in 'slow-moving water' vs 'very slow-moving river water'. We need to respect that both scenarios involve slow water but have opposite algae outcomes

Strategy

For paradox questions, we need to find what's different between the two scenarios that explains the contradiction. Both situations have slow-moving water, but only the moderate low rainfall leads to high algae. Something about extreme drought must create conditions that prevent algae growth despite the slow water. We should look for factors that extreme drought introduces that moderate low rainfall doesn't have

Answer Choices Explained

During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.

This doesn't resolve our paradox. If fewer animals are eating algae during drought, we'd expect algae levels to be higher, not lower. This choice actually makes the contradiction worse rather than explaining it.

The more slowly water moves, the more conducive its temperature is to the growth of algae.

This supports the normal pattern but doesn't explain the exception. If anything, this suggests that extremely slow water during drought should have even more algae, which contradicts what we observe. This choice reinforces the paradox rather than resolving it.

When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.

This talks about what happens to other species when algae levels are high, but our puzzle is about why algae levels are low after extreme drought. This choice doesn't address the timing or conditions we're trying to explain.

Australian rivers dry up completely for short intervals in periods of extreme drought.

This perfectly explains the paradox! If rivers completely dry up during extreme drought, all the algae would die during these periods. Even when water returns and flows slowly (ideal conditions for algae), the algae populations remain low because they were wiped out when the river dried up completely. This explains why we see low algae levels 'following a period of extreme drought' - the populations haven't recovered from being completely eliminated.

Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.

This compares dammed vs. free-flowing rivers but specifically excludes extreme drought periods from this pattern. This doesn't help us understand what's special about extreme drought that causes low algae levels.

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