Logical reasoning PrepTest 102 · Section 3 · Question 4

Question prompt

Scientists analyzing air bubbles Remaining source text redacted.
Why the credited answer is right

Credited answer: D

The notes below walk through why it fits the stem and how to eliminate the rest.

Argument or Facts

Argument

Valid or Flawed

Flawed

Question Type

Weaken Questions

Stimulus Summary

Phenomenon - The ice age atmosphere had a lot of ferrous material and little CO2 Explanation - The ferrous material caused a lot of algae (which absorb CO2) to grow

Answer Anticipation

The question stem asks us about a hypothesis, and hypotheses are generally causal explanations for witnessed phenomena, so we should head into the stimulus expecting that. And we end up getting it. The phenomenon is the high levels of ferrous materials and low levels of CO2 in the atmosphere during the ice age. And the scientists’s hypothesis? That the high levels of ferrous materials caused algae to bloom, and that algae absorbed the CO2 from the atmosphere. However, note that there are no premises about the population of algae - just that it absorbs CO2. So here, that increase is assumed based on the increase in ferrous materials and the decrease in CO2. Since we’re dealing with a hypothesis, there are two primary avenues of weakening it. A correct answer can either present information that undercuts the hypothesis directly (e.g., questioning whether there was an increase in algae), or it can provide evidence for an alternative hypothesis. Let’s keep our eyes peeled for either of those as we read through the answers.

Answer choices

  1. A
    Diatoms are a microscopic Remaining source text redacted.
    Why choice A is not credited
    If anything, the diatoms staying largely unchanged means that what we know about them would have been true during the last ice age, making the argument stronger.
  2. B
    Computer models suggest that Remaining source text redacted.
    Why choice B is not credited
    This answer aligns with the causality in the hypothesis, so this strengthens the argument.
  3. C
    The dust found in Remaining source text redacted.
    Why choice C is not credited
    This is a tempting answer - it seems as if it presents an alternative cause of the algal bloom. However, the stimulus concludes that an “unusually large” amount of ferrous material led to an increase in the population of the algae. This answer doesn’t establish that the other minerals existed in higher-than-normal amounts, so they couldn’t have led to an increase in the algae population (if one even happened - since that’s only mentioned in the conclusion, we don’t know that it did). Thus, this anwer doesn’t present an alternative explanation, and it’s therefore incorrect.
  4. D
    Sediment from the ocean Remaining source text redacted.
    Why choice D matches the stem
    This answer shows that there’s no evidence in an increase in diatom shells from when they die. Since that strongly suggests that there wasn’t an increase in the diatom population, and diatoms are the only example of algae we have, there’s evidence that there wasn’t an increase in the algae population. Since the hypothesis is that the algae population’s increase was responsible for the decrease in atmospheric CO2, this answer undermines the hypothesis and is therefore correct.
  5. E
    Algae that currently grow Remaining source text redacted.
    Why choice E is not credited
    The stimulus had the ferrous materials helped by ferrous materials, not harmed, so this answer doesn’t really add anything to the argument.

What this tests

Question analytics

Based on historical answer selection rates for this question.

Answer choice distribution

  1. A 20%
  2. B 2%
  3. C 17%
  4. D Credited 56%
  5. E 5%

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