OG Gmat Review 11th edition Sentence Correction #59

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His studies of ice polished rocks in his Alpine homeland, far outside the range of preset day glaciers, led Lousi Agassiz 1837 to propose th econcept of an age in which great ice sheets had existed in now currently temperate areas.


a. same
b. in which great ice sheets existed in what are now temperate areas.
c. when great ice sheets existed in what are now temperate areas.
d. when great ice sheets had existed in current temperate areas.
e. when great ice sheets existed in aresa now that are temperate

the correct answe is b because simple past is correct.
WHy can't the past perfect be used. My reason for asking this is the ice had and ended in the past. Does this not qualify for the use of past perfect tense? I am confused on when to use simple and perfect tenses.
Appetite for 700 and I scraped my plate!