GMAT Critical Reasoning (CR) | Re: The prairie vole, a small North American grassland rodent,

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

Which of the following, if true, provides
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