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? ['Prairie vole populations vary dramatically in size from year to year.', 'It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.', 'The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.', 'Snakes, a major predator of young prairie voles, are active only from spring through early autumn.']