Explanation

Reading the question: the prompt leaves some questions unanswered. Cuckoos usually are antisocial parasites. But ani cuckoos are social--are they parasites? The answer to that question doesn't leap out. Furthermore, there is the question of the "living conditions"--what are they?

Creating the filter: of these questions, the most important, we can conjecture, is what exactly these living conditions are. The parasite business is irrelevant, perhaps, and we are talking about the question of antisocial versus social, which will depend on the living conditions. Clarifying that would add further "support" to the "scientists' explanation." So we can use "living conditions" as a basic relevance filter.

Applying the filter: choice (A) gives us genes, not living conditions, so it doesn't pass the filter. Choice (B) looks relevant--it discusses living conditions and resolves the mystery about whether ani cuckoos are parasites. So (B) stays in. Choice (C) appears to be true of all cuckoos, so it does nothing to explain the difference among them. Choice (C) is out. Ditto for (D) and (E). We're left with (B) only.

Logical proof: when you're reasonably certain on a question or running behind on time, you'll want to skip the logical proof step. Applying the negation test whenever possible on Critical Reasoning questions may be too time-consuming; you can skip it selectively to balance time management and accuracy. However, we can apply the negation test to (B). Say that ani cuckoos did not nest communally. In such a case, they might well be brood parasites, like other cuckoos. And in such a case, their living conditions, so far as we know, would not be communal. The negation of (B) would severely weaken the argument, so (B) strengthens the argument. The correct answer is (B).