GMAT Sentence Correction (SC) | Re: His studies of ice-polished rocks in his Alpine home land,

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