I got A, wrong choice. But now I understand why correct choice is D. Let's I try to reason:
(A) dark-shelled snails in a calm inlet with a dark, rocky bottom and many predators => in calm inlet is contradict with the argument, so wrong
(B) dark-shelled snails in a calm inlet with a white, sandy bottom => calm inlet is also wrong
(C) light-shelled snails in an inlet with much wave action and a dark, rocky bottom => should be dark-shell snails
(D) light-shelled snails in a calm inlet with a dark, rocky bottom and many predators => light in calm inlet. Dark, rocky, and many predators prove this species successfully avoid the recognition of predator. Rocky bottom also help snail absorb more heat from sun than sandy bottom. So, correct one.
(E) light-shelled snails in a calm inlet with a white, sandy bottom and many predators => same reason as A, B, and C.
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