Critical Reasoning

The prairie vole, a small North American grassland rodent, breeds year-round, and a group of voles living together consists primarily of an extended family, often including two or more litters. Voles commonly live in large groups from late autumn through to winter; from spring through early autumn, however, most voles live in far smaller groups. The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.

Which of the following, if true, provides the strongest support for the explanation offered ?
A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.
B. Prairie vole populations vary dramatically in size from year to year.
C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.
D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing.
E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn.
oa is e
What's wrong with options B and D?

The conclusion of this argument is the "explanation" we've been asked to support: The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.

The evidence is that "Voles commonly live in large groups from late autumn through to winter; from spring through early autumn, however, most voles live in far smaller groups."

Let's start with why Choice E is correct. We given a good reason to think that young voles would be less abundant in the spring/autumn. They 're killed by snakes, which strongly supports "young vole mortality" as a reason for seasonal variation in groups size. The explanation/conclusion is strengthened.

You've asked about choice B and D. Choice B says that "Prairie vole populations vary dramatically in size from year to year." Interesting . . . but irrelevant. We only want to know about the vole population change from season to season within a year. This choice doesn't address that.

Choice D tells us the winters are harsh. That would be nice to know if we were trying to explain why vole population decreased in the winter. But the population decreased in the spring. Furthermore, there is no mention of the young vole population, and they're the "star" of the conclusion.

I'm available if you'd like any follow up.