Sentence Correction

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 what are now temperate areas

(A) in which great ice sheets had existed in what are now 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

OA: B

I have confusion between B and E. The official explanation says, 'Now is an adverb and should be placed just after the verb are'.

I couldn't get this explanation. Also are there any other specific rules too (like the one above) that govern the placement of adverb.

Please help.