Sentence Correction

His Studies of ice-polished rocks in his alpine homeland, far outside the range of present day glaciers, led LOuis Agasiz in 1837 to propose the concept of an age in which great ice sheets had existed in now currently tempereate areas

a. same
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 sheers existed in areas now that are temperate

The correct answer is B. I chose A. However, looking back I can see that my answer was redundant. BUt as far as the verb tense I am very confused still. I know that in past perfect - if the does not exist anymore and interrupted by something else in past is indeed past perfect. THis requires HAD. HOw did I mess up on the verb tense?

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