Logical reasoning PrepTest 138 · Section 4 · Question 17

Question prompt

Several years ago, as Remaining source text redacted.
Why the credited answer is right

Credited answer: A

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

Strengthen Questions

Stimulus Summary

Random detail - Gypsy moths depend on oak leaves for food Problem - There are toooooooo many gypsy moths Entomologist Solution - Introduce a fungus that’s poisonous to gypsy moths Results - The gypsy moth population has significantly declined Conclusion - The poisonous fungus caused the moth population decline

Answer Anticipation

A Strengthen question with a causal conclusion? You should hop immediately to seeing if this is based on a correlation and, if it is, start to look for an answer that falls into one of the common categories. Here, the Entomologists conclude that the poisonous fungus caused the decline in the gypsy moth population because that decline happened after the fungus was introduced. That’s a Correlation/Causation flaw, so we can use our common answer types: Eliminate an alternative cause Find more examples of cause and effect going together Introduce a control (no cause, no effect) Explain the causality With all that noted, we can actually dig a little deeper here! When an alternative cause is suggested in the stimulus, it’s frequently relied on for the right answer. Here, the stimulus - for seemingly no reason - states that gypsy moths depend on oak leaves for food. That means an alternative cause of the decline in the gypsy moth population is a decline in available oak leaves. Let’s keep an eye out for an answer that eliminates that as a possibility!

Answer choices

  1. A
    A strain of gypsy Remaining source text redacted.
    Why choice A matches the stem
    This answer functions similarly to the control group answer. By showing that the gypsy moths for whom the cause isn’t relevant (they’re immune to the fungus) also didn’t see the effect (their population didn’t decline, or at least it didn’t decline as much as other gypsy moths), this strengthens the argument. Note that it has nothing to do with oak leaves - the tendencies on the exam don’t always pan out!
  2. B
    The fungus that was Remaining source text redacted.
    Why choice B is not credited
    The impact of the fungus on other species of insect is out of scope of an argument about whether it caused a decline in the gypsy moth population.
  3. C
    An increase in numbers Remaining source text redacted.
    Why choice C is not credited
    If anything, this answer suggests a potential alternative cause - a surge in the predator species. If there was such a surge around this time, that might have caused the population decline instead of the fungus.
  4. D
    In the past several Remaining source text redacted.
    Why choice D is not credited
    This answer brings in the oak leaves, matching our anticipated specific answer! However, in stating that the oak tree population has declined, this answer suggests that the gypsy moth food source has gone down, suggesting an alternative cause for the moth population decline. This identifies an alternative cause - it doesn’t eliminate one - so it’s on the Weaken side, not the Strengthen side.
  5. E
    The current decline in Remaining source text redacted.
    Why choice E is not credited
    This answer shows counterexamples - no cause, same effect. If the population went down where the fungus wasn’t introduced, that suggests something else led to the decline in the population, weakening the argument.

What this tests

Question analytics

Based on historical answer selection rates for this question.

Answer choice distribution

  1. A Credited 71%
  2. B 15%
  3. C 9%
  4. D 1%
  5. E 3%

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