D
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 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 a seasonal variation in mortality among young voles. Which of the following, if true, provides the strongest support for the explanation offered?
A: Prairie vole populations vary dramatically in size from year to year.
B: It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.
C: The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.
D: Snakes, a major predator of young prairie voles, are active only from spring through early autumn.