[求助]白勇Section 6-18

18. His studies of ice-polished rocks in his Alpine home-land, 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

答案：B

请教为什么不能用when引导？