Explanation
No need to look at the answer choices here: we know what
the correct answer is going to be. The author is asking us what the theories
are for--that's the point of the passage as a whole, which we know. The theories
are trying to explain how birds are able to use magnetism to navigate long
distances. We look for that in the answer choices. (B) fits.
Some of the other answer choices are in the ballpark but have faults. (A) is out because there's little to no discussion of landmarks
through the passage. (C) has a similar problem, in
that species are not compared. (D) expresses a fact
that predates the theories, not something the theories are trying to explain.
(E) expresses part of the idea of theory 1, but not
the purpose of theory 1 and theory 2.
The correct answer is (B).
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