Sentence Correction

I've been scouring the web and haven't got a convincing idea as to how GMAT treats when, in which and during which:

In (1) Are when and in which interchangeable? I thought when refers to a specific time and not time period like "age"?
Could "during which" be a candidate as well - since age is technically a time period?
In (2) I've added a SIXTH answer choice that uses during which instead of era during which. Is F also correct since During which also refers to a time period?

1) 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 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
OA : B



2) Tom Bradley was mayor of Los Angeles from 1973 to 1993, an era when the city had
transformed from a collection of suburban neighborhoods to the second-largest city in
the United States
.
A. an era when the city had transformed.
B. an era during which the city was transformed.
C. an era that transformed it.
D. during which era the city transformed.
E. during which the city was transformed
F. During which the city was transformed.

OA : B

Balrog1978 wrote:I've been scouring the web and haven't got a convincing idea as to how GMAT treats when, in which and during which:

In (1) Are when and in which interchangeable? I thought when refers to a specific time and not time period like "age"?
Could "during which" be a candidate as well - since age is technically a time period?
In (2) I've added a SIXTH answer choice that uses during which instead of era during which. Is F also correct since During which also refers to a time period?

1) 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 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
OA : B



2) Tom Bradley was mayor of Los Angeles from 1973 to 1993, an era when the city had
transformed from a collection of suburban neighborhoods to the second-largest city in
the United States
.
A. an era when the city had transformed.
B. an era during which the city was transformed.
C. an era that transformed it.
D. during which era the city transformed.
E. during which the city was transformed
F. During which the city was transformed.

OA : B

Your first question : when vs in which.

I hope that you are reading MGMAT SC guide.

As per that guide,in which and when are interchangeable,whereas where an in which are NOT!.

where should specifically refer to a place [a country,region,or an area].It cannot refer to hypothetical place such as meeting,party etc.

regarding during which,there is no such rule.In q-2 ,during is a Preposition!. which is the object of this preposition.Which is referring to era!

The questions - both 1 and 2-have caught you napping looking for false splits.The real split is tense!

q-1

had existed is incorrect in A and D [read MGMAT SC for more on tenses].We want same tense as led[past tense] in this sentence for these 2 things are happening in 2 totally different frames of time; they are totally unrelated!

B,C,and E use correct tense [existed]

C contains a new tense error - it says now [with temperate] and at the same time uses past tense were.It should be are.

Eliminate C

left with B and E

Positioning of now should be closer to the verb it modifies to avoid ambiguity [are].There are two verbs existed and are.To avoid ambiguity we want now to be as close as possible to "are" to keep the meaning of the sentence same.

In question 2

an era.... in US [this is an appositive/an absolute phrase/noun phrase]

It has NO verb!

The error in this question again is tense error.

Had transformed is incorrect,for the sentence uses past tense in Independent clause [Tom... 1993],with main verb was in past tense.

the era could NOT have begun prior to period in which this guy became mayor!

C -meaning issue.Read this carefully! This is very funny! can an era transform the city?? Logically a person or his actions can transform the city [LA].

Eliminate C

We are left with B,D,and E

What does which refer to in E? it could refer to LA,1973 to 1993,or 1993 [none of them make much sense]

F too is wrong for same reason.

We are left with B and D

You can clearly see that between B and D B is clearly more clear. D does not clearly state what it is modifying and also has a weird [active voice] form of verb as in C.

Also,In such sentences try to prefer passive voice.I believe that there is a question similar to this one in OG that prefers passive voice for the same reason.

Hope this makes it clear.

So if we have another answer choice like this:

6) During which the city transformed

this would ALSO be a correct answer choice?
(removed was from (5) as well as assuming that era during which and during which both are correct)