Explanation
We are asked for something "useful to know" in evaluating
the second theory. The most useful thing to know will always be something that
is critical to know, if there is such
a thing, so we can always start with that. We can evaluate each answer choice
with that question: is this critical to whether or not theory 2 is correct? (A)
mixes magnetite grains and retinas, details from
theory 1 and theory 2, in a jumble. So (A) is out. (B) may
be critical, or at least relevant. (C) is not critical to whether the theory is
correct; it may impact whether the theory is understood, but the passage concerns
only whether the theory is correct. So (C) is out. (D) has
promise. (E) is another jumble, because wandering
macrophages are not relevant to theory 2, only theory 1. We are down to (B) and
(D). Which is more critical? It's not critical that other birds have retinas
that are identical to those of
robins, only whether they are identical in the relevant aspect. Their retinas
could be of a different color, or larger, but so what? And that leads us back
to the cryptochrome. The passage says that "scientists
believe" that cryptochrome processes magnetic
information. If that's not true, the theory will fall apart. If it is known to
be true, then the theory is a little stronger.
The correct answer is (D).
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