13. His studies of ice-polished rocks in his Alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.

(A) in which great ice sheets had existed in now currently temperate areas

(B) in which great ice sheets existed in what are now temperate areas

(C) when great ice sheets existed where there were areas now temperate

(D) when great ice sheets had existed in current temperate areas（B）

(E) when great ice sheets existed in areas now that are temperate

为什么选B，请解释一下？ in后面可以直接跟What? A和E分别错在哪？

首先， the age in 不能改变。

what are now temperate areas相当于the place which

我的想法是这样的：

首先，age是说一个时期。所以用in which比用when好。when通常指某一时间点。这样先砍掉c,d和e。a用过去完成时不好，因为它在句中没用针对某一过去时。