Nice question. The power split on this problem is parallelism, but I haven't seen much discussion about it because it's a bit hard to see.

You have to pay attention to the core of the sentence to find the parallelism: "A low dose of aspirin usually suffices to block production of thromboxane but not seriously interfering with the production of prostacyclin." By taking out the modifiers we can see the parallelism and more easily notice the error - "suffices 'to block' ... but not 'interfering'." We can see that we need the infinitive form of interfere (remembering that the GMAT allows us to drop the leading 'to') to be parallel. A, B, and E have incorrect forms of "interfere" and get eliminated. That leaves C and D, but D gets eliminated because it's less concise and it's unclear (D really should be written "which is a substance used to promote...").

KW
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Just curious is a Noun + noun modifier preferred over a which clause?

It's hard to say, and the each case would be evaluated on its own, but

thromboxane,
I)which is a substance that (...)
II)a substance that (...)

(if everything else in the sentence is correct) the second case is better then the first one, just because it's more concise: you do not need "which" to refer back to "thromboxane" , you can just use an appositive modifier.

Hope it helps

Couple points of clarification here.

The sentence is about aspirin and we DO want non-essential modifiers after thromboxane and prostacyclin. 'Which' modifiers create non-essential clauses but so do appositives set off by commas. "A substance that promotes blood clotting" is a non-essential appositive and correct in the sentence.

That said, the "which" modifiers aren't great. Since we have a "which" modifier at the end of the sentence, it can be a bit confusing to the reader to have multiple "which" clauses in the sentence unless they have the same structure.

KW
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(D) which is a substance to promote blood clotting, but does not seriously interfere

- My reason for eliminating D apart from usages of "which" and "to promote" is Substance needs Promotes (singular) not Promote (Plural).

(C) a substance that promotes blood clotting, but does not seriously interfere.

C clearly states substance promotes. Hence, between C & D, C is correct.

"But" belongs to the FANBOYS group and connects two Independent clause.
Here we are using But and there is no IC after it which gives us another reason to eliminate option A, B , E
Parallelism is also the issue here. As mentioned by KyleWiddison

But While selecting Between C and D i got confused and assumed that which is referring to the "thromboxane " and whole Which modifier structure should be parallel to the other Which Modifier structure. Because of this thinking i endup choosing option D over C .

Can you please Explain why C is more preferable ? is it because of "to promote " doesn't convey the right meaning ? or is it we prefer Noun + Noun Modifier over any other modifier because of its power to modify any item in the sentence ? or is there any reason which is yet not mentioned or i am not aware of ?

Please help !!!
GMATNinja, aragonn, GMATNinjaTwo, hazelnut, generis
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GMATNinja , EMPOWERgmatVerbal , Bunuel, GMATNinjaTwo , ScottTargetTestPrep ,

Hello. I always thought that WHICH is supposed to modify the preceding word (touch rule). Since the beginning of the underlined portion is supposed to describe thromboxane, isn't a "WHICH" modifier necessary here?

That's the reason I chose D over C and I totally missed the parallelism error in D. But if D were "which is a substance THAT PROMOTES blood clotting, but does not seriously interfere" ----> would this be correct?

Please help. I've been stumped in this question for almost an hour already! :P

Hi Diwabag,

One key to correctly answering a Sentence Correction question is to avoid getting locked into your own idea of how the sentence should be written. You have to keep your mind open to a structure that you weren’t expecting and meanings different from the one that you expected the sentence to convey. In this case, while a relative clause beginning with “which” would work, there are other types of modifiers that would also work, and as it turns out, the correct version uses an appositive rather than a relative clause.

Also, be aware that Sentence Correction question writers may put familiar structures, such as clauses beginning with “which,” in incorrect choices and less familiar structures, such as appositives, in correct answers. So, you have to be ready to see structures that you didn’t expect to see in correct Sentence Correction answers. Be careful about gravitating toward the familiar, and make sure you consider entire choices, so that you notice any flaws that may accompany familiar, correct-sounding structures.
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thank you expert for good explanation.
if in choice c, we use "which" rather than "that" the sentence is wrong
a substance, which promotes blood clotting

this meaning will be wrong because the sentence means that any substance promotes.
if we cut off which, and the meaning is good, which is correct. if we cut off that , and the meaning is wrong, "that" is correct

if we see 'that', think that there are more than 2 kinds of noun preceding "that clause". if we see "which, think that there is only one kind of noun. this is the way to understand the meaning of which and that clause.

in choice c, if we substitute " that" with "which", we can omit which clause.

In laboratory rats, a low dose of aspirin usually suffices to block production of thromboxane, a substance (which promote)

there is not phrase in the parenthesis, and the sentence is wrong.

i feel hard to differentiate that and which clauses

In laboratory rats, a low dose of aspirin usually suffices to block production of thromboxane, which is a substance that promotes blood clotting, but not seriously interfering with the production of prostacyclin, which prevents clotting.

(A) which is a substance that promotes blood clotting, but not seriously interfering
(B) a substance that promotes blood clotting, but not seriously interfering
(C) a substance that promotes blood clotting, but does not seriously interfere
(D) which is a substance to promote blood clotting, but does not seriously interfere
(E) which is a substance that promotes blood clotting, but not a serious interference


Why option D is incorrect? somebody please help.
Thanks

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