Sixty-five million years ago, according to some scientists, an asteroid bigger than Mount Everest slammed into North America, which, causing plant and animal extinctions, marks the end of the geologic era known as the Cretaceous Period.
A. which, causing plant and animal extinctions, marks
B. which caused the plant and animal extinctions marking
C. and causing plant and animal extinctions that mark
D. an event that caused plant and animal extinctions, and it marks
E. an event that caused the plant and animal extinctions that mark
OA is E
Could you tell me E?
Thanks

"which" indicates a noun modifier, which is required to be placed as close as possible to the noun it modifies. This typically means it must be placed right next to that noun, though there are occasional exceptions to the rule (there are always exceptions!).

In this case, North America did not mark the end of the era. The act of the asteriod slamming into North America marked the end of the era. So we can't use a noun modifier here b/c we aren't just referring to a simple noun.

Elim B for same reason.
C starts with "and" - that should make the two parts (before and after the "and") parallel, but they're not in C (and this isn't the intention of the sentence anyway - we want to say that one event led to the next).
D and E both correctly start with "an event that caused" - yes, that's both the original intent of the sentence and it's grammatically correct.
D introduces another "and" towards the end, which again dictates parallelism (which is not present here) and again mars the original intent of the sentence - one event led to the next, rather than two events being side-by-side parallel
E correctly indicates that the event caused the extinctions and those extinctions mark the end of the era.

Stacey's explanation about D and E makes perfect sense.

However, the reason I didn't pick E was because "an event" (singular subject), doesn't match up with "mark" (plural verb). Am I not picking out the subject/verb correctly?

Thanks.

well, choice e says:
an event that caused the plant and animal extinctions that mark...

the boldface part is an 'essential modifier': a relative clause that MUST modify the immediately preceding noun, in this case 'extinctions'. these sorts of modifiers - 'that' without a comma - follow exactly the same rule that you (hopefully) use for 'which' following a comma: as stated above, they must modify the immediately preceding noun.

since the immediately preceding noun in this case is 'extinctions', you need a plural verb.

incidentally, i find the use of the present tense 'mark' to be a bit bizarre; in my opinion it would make mone sense to say 'marked' instead, as the events we're talking about are clearly in the past.

Hi Instructors,

Just a small question regarding this SC..

in choice E we have : THE plant and animal extinctions

Could you please explain if this usage correct ???

thanks in advance
Anon

well... it's in the correct answer, so it must be correct. so there's your answer on that one.
(not meant to be sarcastic; the correct answers to official problems are the best source of information about what the gmat considers to be correct/incorrect in tcerms of usage and diction.)

in general, this seemingly extraneous use of 'the' is indicated if the sentence refers to very specific events, items, etc., particularly if those events/items/etc are assumed to be known to the reader. more so if the events/items/etc form a complete set.
in this problem, these things all make sense. (as for the last point, the form of the correct sentence allows us to infer that the asteroid impact caused all of the plant and animal extinctions that ended the cretaceous period.

if you're worrying about how you'll ever distinguish between sentences that need this 'the' and sentences that don't, then don't worry: you'll never have to make that decision. the gmat won't make you decide between two choices that are both grammatically correct based on such a subtle semantic difference, so there will be some other grammatical error that disqualifies the wrong answers (as in this problem).
so just realize that this form is acceptable, DON'T use it to disqualify an answer choice, and go from there.