70. 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

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

are temperate

选B，A里面的now currently重复，但是关于时态有点疑惑，为什么要用existed而不用had existed? 不是在过去存在吗。。。

劳烦童鞋们了。多谢。。