Biological functions of many plants and animals vary in cycles that are repeated every 24 hours. It is tempting to...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Weaken

MEDIUM

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Notes

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Biological functions of many plants and animals vary in cycles that are repeated every 24 hours. It is tempting to suppose that alteration in the intensity of incident light is the stimulus that controls these daily biological rhythms. But there is much evidence to contradict this hypothesis.

Which of the following, if known, is evidence that contradicts the hypothesis stated in lines 2-5 above?

Human body temperature varies throughout the day, with the maximum occurring in the late afternoon and the minimum in the morning.

While some animals, such as the robin, are more active during the day, others, such as mice, show greater activity at night.

When people move from one time zone to another, their biological rhythms adjust in a matter of days to the periods of sunlight and darkness in the new zone.

Certain single-cell plants display daily biological rhythms even when the part of the cell containing the nucleus is removed.

Even when exposed to constant light intensity around the clock, some algae display rates of photosynthesis that are much greater during daylight hours than at night.

Solution

Passage Analysis:

Text from Passage	Analysis
Biological functions of many plants and animals vary in cycles that are repeated every 24 hours.	What it says: Many plants and animals have biological functions that follow 24-hour cycles What it does: Sets up the basic scientific observation about daily biological patterns What it is: Scientific observation/background fact Visualization: Plants/animals → Daily cycles (every 24 hours) → Functions repeat
It is tempting to suppose that alteration in the intensity of incident light is the stimulus that controls these daily biological rhythms.	What it says: People naturally think light changes control these 24-hour biological cycles What it does: Presents a logical hypothesis that connects light intensity to the biological cycles mentioned earlier What it is: Hypothesis being presented
But there is much evidence to contradict this hypothesis.	What it says: There's a lot of evidence that shows the light hypothesis is wrong What it does: Directly challenges the hypothesis, setting up that we need contradictory evidence What it is: Author's counter-claim

Argument Flow:

"We start with a scientific fact about 24-hour biological cycles, then see a reasonable hypothesis about light controlling these cycles, and finally get told this hypothesis is contradicted by evidence."

Main Conclusion:

"The hypothesis that light intensity controls daily biological rhythms is contradicted by evidence."

Logical Structure:

"This is actually setting up for the question rather than making a complete argument. The author presents a hypothesis (light controls biological cycles) and claims there's contradictory evidence, but doesn't provide the evidence. The question asks us to identify what would count as contradictory evidence."

Prethinking:

Question type:

Weaken - We need to find evidence that contradicts the hypothesis that light intensity changes control daily biological rhythms

Precision of Claims

The hypothesis specifically claims that 'alteration in the intensity of incident light' is the stimulus that controls 24-hour biological cycles in plants and animals

Strategy

To weaken this hypothesis, we need evidence showing that biological rhythms can exist or persist even when light intensity changes are absent, altered, or don't match the 24-hour pattern. We're looking for scenarios where the light-rhythm connection breaks down.

Answer Choices Explained

Human body temperature varies throughout the day, with the maximum occurring in the late afternoon and the minimum in the morning.

This actually supports the existence of 24-hour biological cycles, but it doesn't contradict the light hypothesis. We don't know whether this temperature variation occurs under constant light conditions or not. This choice simply describes a biological rhythm without testing whether light controls it.

While some animals, such as the robin, are more active during the day, others, such as mice, show greater activity at night.

This describes different activity patterns but doesn't contradict the light hypothesis. Both robins and mice could still be responding to light intensity changes - robins being active when light is bright, mice being active when light is dim. This doesn't test what happens when light conditions are controlled.

When people move from one time zone to another, their biological rhythms adjust in a matter of days to the periods of sunlight and darkness in the new zone.

This actually supports the light hypothesis rather than contradicting it. If people's rhythms adjust to new patterns of light and darkness, this suggests that light changes do influence biological rhythms, which is exactly what the hypothesis claims.

Certain single-cell plants display daily biological rhythms even when the part of the cell containing the nucleus is removed.

This shows that biological rhythms can persist without the nucleus, but it doesn't address whether light intensity still controls these rhythms. The cells could still be responding to light changes even without their nucleus. This doesn't test the role of light.

Even when exposed to constant light intensity around the clock, some algae display rates of photosynthesis that are much greater during daylight hours than at night.

This directly contradicts the hypothesis by showing that biological rhythms persist even when light intensity is kept constant. If light intensity changes controlled these rhythms, then constant light should eliminate the day-night variation. Since the algae maintain their 24-hour cycle despite constant light, this proves that light intensity changes are not the controlling factor.

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