Premise：voles live in large groups from late autumn through winter, live in smaller groups from spring through early autumn.
Conclusion：the mortality among young voles can explain the seasonal variation in group.
Logic: because the cold/food, the voles dead, they are together form late autumn through winter. 选项：
A.提到young voles 但是时间段the spring and early summer 不对。Pass
B.无关。
C.spring食物丰富，时间段不对。
D.时间点没有严格对应 late atumn 没有E严格。
E.bingo 时间完全对应。简单推断也向到mortality，所以选E