'when' vs. 'in which'

Valentino99 wrote:Hello,

Is there any rule or advisable guidance about usage of 'when' vs. 'in which'.

Example OG 13 Q:

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 areas
E) when great ice sheets existed in areas now that are temperate


ans: B

thanks!

Here we need to mention what that concept consists of , we cant just say he proposed a concept in 1837, we need to mention about the details of that concept as well.
IMO : B

either "in which" or "when" can be used to modify a time/event. no difference here - red herring

1st split is "existed" vs "had existed", we need former one. -> A,D out(let me know in case you need an explanation)
C is not only awkward but completely wrong. it says "where there were areas now temparate" that means as if areas that are now temparate were somewhere else - wrong because areas do not move from one place to another
E used modifier "now" incorrectly. "now" should modify "are temparate" to convey areas are now temparate but were extremely cold when ice-sheets existed. e) nonsensically says "sheets existed in areas now" - "now" modifies "existed", misplaced modifier - and separates "areas" from its modifier "that"
B is correct. see placement of "now".

Note: common notion is that "what" is often used incorrectly but here it is used correctly. "what" is used for "interrogation". asking "what are now temprate areas" would give us answer as some place/area (say alaska) that can be substituted in place of above interrogative clause, for example, ice sheets existed in alaska.