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?

• AIt is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.  
• BPrairie vole populations vary dramatically in size from year to year.  
• CThe prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.  
• DWinters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing.  
• ESnakes, a major predator of young prairie voles, are active only from spring through early autumn.