求助OG13 SC-72 时态问题

魔力molise

72. 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 area


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


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


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


句子中age 的时间个人觉得发生在propose之前，也就是过去的过去，为何时态不是过去完成时而是一般过去时呢？
谢谢回答！

bejamin1111

前两天刚有人问了这个提，你去搜下