GMAT - Critical Reasoning - Critical Reasoning - Strengthen the argument
The prairie vole, a small North American grassland rodent, breeds year round
The prairie vole, a small North American grassland rodent, breeds year round, 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 through 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 the strongest support for the explanation above?
A) It is the spring and in the 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 prairie vole's 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.
The conclusion of the passage is; among the young voles, seasonal variation in group size can perhaps be explained by a seasonal variation in mortality. the passage tells us that voles are grassland rodent, they breed year round, they live in groups. The group size is the largest from late autumn to winter. The size of the group is smaller from spring to early autumn.
The question asks us to choose an answer that strengthens the conclusion.
A) This is incorrect as the passage tells us that the vole group size is small during spring to early autumn. Summer comes after spring and before autumn, which means the size is small even during summer.
B) This has already been mentioned clearly in the passage, so this answer choice is not adding any new premise to strengthen the conclusion and hence is eliminated.
C) If the voles depended on broad-leaved plants for their food, which is abundant in spring, then the rodents must have a high mortality rate and hence, a larger group during spring. But the passage tells us that during spring the groups are small. This answer is also eliminated.
D) This weakens the conclusion as the passage provides support for larger groups during winter. And if winters were harsh then the voles mortality rate would drop, hence we eliminate this answer choice.
E) This is the correct answer as rodents are prey for snakes and if they are active from spring through early autumn, the same time when the voles group size reduces, it means that they are being eaten by the snakes. This answer choice strengthens the conclusion and hence is the correct choice.