In OG 11th

59. 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* concept of an age in which great ice sheets' had existed in now currently temperate areas.

A) in which great ice sheets had existed: 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

Why does it use past tense "existed", not past perfect "had existed"?