A at some time in the past our ancestors suffered an event, greatly reducing their numbers ------ A hyphen is not the tool to connect two ICs.
B that at some time in the past our ancestors suffered an event that greatly reduced their numbers -------- ‘that ’ is the right connector ---correct choice.
C that sometime in the past our ancestors suffered an event so that their numbers were greatly reduced ------- altered notion; it looks as if the ancestors suffered a calamity in order to reduce their numbers.
D some time in the past our ancestors suffered an event from which their numbers were greatly reduced -------- conjugation of two ICs with hyphenation is wrong.
E some time in the past, that our ancestors suffered an event so as to reduce their numbers greatly, ------ same as in D

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In answer choice (A) there is a problem with modification. (A) is implying that our ancestors greatly reduced their own numbers (this is incorrect because it was the event that greatly reduced ancestors). When we have an independent clause followed by a participle phrase (one that starts with a gerund and serves as an adjective clause), the participle phrase modifies the subject of the sentence.

In non-grammarese: 'ancestors' is the subject of the independent clause, 'at some time...' and because of the comma after event, we have the incorrect meaning. It was not the ancestors but an event that 'reduced their numbers.'

Therefore, we want to make sure that it is clear that 'event' is 'greatly reducing the numbers.' One way to fix that is by using the relative pronoun 'that.' In (B), we have 'an event that greatly reduced their numbers' that does a good job of correcting the error in (A).

Therefore (B) is the answer.
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Christopher Lele
Magoosh Test Prep

I had thought on the same lines initially (that X and that Y) - but closely looking at these two choices you would notice there is a clear change of meaning in the second choice (C)

we can replace 2nd "that" in B with "which" and sentence makes same sense. so its not ( that X and that Y)
B that at some time in the past our ancestors suffered an event that greatly reduced their numbers

"so that" in this means "because of". It conveys as if the ancestors suffered an event to cause a reduction in their own numbers.
C that sometime in the past our ancestors suffered an event so that their numbers were greatly reduced

Hope it makes sense.
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Thanks a Lot PUNEETSCHDV U saved my day... this is the best explanation of the question available....

There are two pending items that need to be discussed in detail.
Usage of '--' and ' at some time'




Can some guru confirm if below points are correct:

some time: Unspecific time . I want to visit chicago sometime this year
At some time in the past : during some indefinite time in the past
at sometime in the past: same as above
some time: some is an adjective referring to time, quite a while
I’ve been spending some time thinking about income inequality for a piece I’m writing.
sometimes : occasionally

some time in the past -> is wrong ??


Regarding usage of '--':
Check the below link from grockit:
http://grockit.com/blog/gmat/2011/08/01/gmat-sentence-corrections-colons-and-dashes/
So if i interpret it correctly 'DASH'es quite flexible, so "-that at some time in the past " is same as "-at some time in the past"...
can some one confirm this..
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Can you see if you can post an official example where above kind of construct is valid. It would be interesting to see.


Actually if you read the OE (this is a OG question), it says: The agent or cause of reducing is unclear.
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Sure no problem; when I had requested:

Can you see if you can post an official example where above kind of construct is valid.

By official example I meant questions from Official guides, GMATPreps etc. (these are called "official sources"), not MGMAT guides.
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Ashish
GMAT - 99th Pecentile, MBA-ISB
Faculty @ EducationAisle
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Hi,
I am bit confused about your explanation, I thought that a participle phrase (gerund) will modify the entire independent clause - in this case the fact that "our ancestors suffered an event"
Example (Taken from MGMAT just to make sure I don't introduce something estrange):
Crime has recently decreased in our neighborhood, leading to a rise in property values

I may be missing something in your explanation, so it will be great if you can expand a bit more on it.

I originally had chosen A, not thinking about the required parallelism: Anthropologist believed that x - that y. I didn't think it made sense to make those two clauses parallel because of the lack of 'and', and thus I figured that the clause after the dash ('=') was just emphasizing the definition of a 'population bottleneck'.

I hope this thread is not to old.

Thanks,