Our ancestors (OG13 97)

alex.gellatly wrote:Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"—at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.
(A) at some time in the past our ancestors suffered an event, greatly reducing their numbers
(B) that at some time in the past our ancestors suffered an event that greatly reduced their numbers
(C) that some time in the past our ancestors suffered an event so that their numbers were greatly reduced,
(D) some time in the past our ancestors suffered an event from which their numbers were greatly reduced
(E) some time in the past, that our ancestors suffered an event so as to reduce their numbers greatly,

"and thus our genetic variation" suggests we need NOUN before AND - so, GREATLY REDUCED in C/D/E wrong

A/B
Problems in A:
(1) REDUCING modifies entire previous clause but fact is its the result of AN EVENT
(2) "our ancestors suffered an event" is an independent clause but not connected with other parts of sentence with proper connector (making it a fragment)
(3) "at some time in the past" is a prep modifier - we do not very clearly understand whether it modifies "population bottleneck" or our ancestors. By placing this phrase after THAT clause, sentence avoid such ambiguity

IMO B

what s the OA??

Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"-at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.
(A) at some time in the past our ancestors suffered an event, greatly reducing their numbers
(B) that at some time in the past our ancestors suffered an event that greatly reduced their numbers
(C) that some time in the past our ancestors suffered an event so that their numbers were greatly reduced,
(D) some time in the past our ancestors suffered an event from which their numbers were greatly reduced
(E) some time in the past, that our ancestors suffered an event so as to reduce their numbers greatly,

Rule 1:

COMMA+VERB-ING modifies the preceding clause and retains the subject of that clause.

our ancestors suffered an event, greatly reducing their numbers.
--> It means that our ancestors themselves have reduced their numbers. this clearly changes the meaning of the sentence. ==> Eliminate A.

So that:
We have studied lot so that we can have better score.
We do something so that we can have something.
Now check the option C.
ancestors suffered an event so that their numbers were greatly reduced.
--> It means that ancestors suffered--willingly--an event to reduce their numbers. It changes the intended meaning of the sentence. ==> Eliminate C.

Unnecessary use of passive in the option D. Besides, option D is wordy. ==> Eliminate D.

Option E has the same problem as it is in C. Besides, the placement of "that" is in wrong place.
==> Eliminate E.

Answer is B.