Explanation
The last sentence of the passage, as we've discussed,
expresses the view that there are two possible explanations for the presence of
the two similar fish in the example. The correct answer to this question will
support that view, so it will reinforce the idea that either explanation could
be correct. It will definitely not rule out one explanation or the other,
although it might simply emphasize that one of the explanations is possible.
That, in fact, is exactly what choice (B) does; it indicates that abundance of
food can allow two species that occupy the same "spot" in the puzzle structure
of a community to coexist with each other. Another virtue of choice (B) is that
it says that the aphid species tend not to coexist, so in one breath it gives weight
to both explanations offered by the author. Answer choice (C), by contrast,
gives weight to the idea that only one of the two possibilities presented by
the author is possible. Answer choice (D) strengths the wrong argument--it's the
initial idea of assembly rules. Choice (E) is irrelevant because it concerns
two species that are functionally similar but differ functionally in a respect
that may be critical. Choice (A) is basically irrelevant.
The correct answer is (B).
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