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#8 - Monarch butterflies must contend with single-celled

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Could you please clarify as to why C is incorrect and D is correct?
Robert Carroll
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The author concludes that a difference in parasitic infection rates between migrating and non-migrating butterflies is due to migration. Why couldn't it be the other way around? In fact, the stimulus even says that infected butterflies can have deformities that interfere with flight. It makes at least as much sense as the author's conclusion to think that instead the butterflies that couldn't fly right didn't migrate, so the difference in infection rates is the cause of differences in migration rather than vice versa. This precisely is answer choice (D).

The size of the population does not affect the percentage infected, so answer choice (C) is itself confusing numbers and percentages. Percentages are relevant here, and it was not a flaw in the reasoning to stick with percentages. There is no need to provide additional information about raw numbers.

Robert Carroll