don't need a which. hence B and C. promotes and interfere are parallel. hence choose C.

answer should be C only, in B if u see "ing form in interfering" is not required and also if we had a sentence like "but also seriously" then it would have been a awkward sentence structure.........we are not being tested any idiom of sort "not x but y"......

Thanks Guys......Can we say that Does not played some key role in making an answer choice leaving the parellism by promotes....

with out does not i think its meaning less to say the sentence......

Thanks for posting this - I love this question! Mainly, I love it because it shows that subtle way that the GMAT likes to create difficulty. One of the more interesting ways that it's been adding that difficulty lately is by showing you what you want to see, but having something else that makes it wrong.

Here, you have:

...a low dose usually suffices to block production of thromboxane, _______________________________

And three of the answer choices start with "which is a substance...". That's perfect - as you've no doubt studied, "which" as a modifier must modify the word that comes directly next to it, and in this case "thromboxane" is, indeed, a substance, so "which" is a perfectly apt modifier here.

What's tricky, however, is that each of the "which" answers breaks parallelism in what comes next. Four of the five answer choices use the phrase:

...a substance that promotes blood clotting

And we need to follow that by saying that the substance does not seriously interfere with another substance. Because we have the presence of "that promotes", we need the second half to be parallel:

...a substance that promotes X but does not interfere with Y

That's because the pronoun "that" needs to assign to both verbs "promotes" and "does not interfere", or the second verb doesn't have a clear subject.

As it turns out, the only sentence that does this correctly is C:

"a substance that promotes blood clotting, but does not significantly interfere..."

And C doesn't have the pronoun "which" in it. But "which" would be perfectly valid in a choice that says:

"which promotes blood clotting, but does not significantly interfere..."

The lesson here - don't get so attached to one way of phrasing a sentence, or to a particular common-GMAT-trigger that you want to see, that you fail to see the real decision point. The GMAT is extremely well-written. It knows that you love certain rules (like this one for "which", or particular common-usage idioms) or have fallen into preferring tricks ("being" is usually wrong), and it will use those as bait. You need to be systematic in your thinking and be sure to only eliminate clearly-wrong answers first, then start moving to other decision points.

Thanks Brain for your explanation!!!! Now I got the point you intended to say......

Hey Brian,

Great Explanation!! Thats exactly what i was thinking, word "BUT" (a parallel marker)

2 questions:---

question 1:-

Op E:-- Which is a substance that promotes blood clotting, but not a serious interference

If we change this option like this:

Which is a substance that promotes blood clotting, but DOES not seriously interfere

Is that gonna be correct??? i think yes..correct me if m wrong pls???

Question 2:--

just wanna clear my concepts related to parallelism:

a substance that promotes blood clotting, but does not seriously interfere----> okay

if we use AND instead of BUT, (i know BUT is necessary and we can't replace with AND as contrast is needed, please forget that at this moment)

a substance that promotes blood clotting, AND does not seriously interfere

I believe that now the above written sentence is NOT okay...its a violation of THAT and THAT parallelism....m i right??? in case of AND we have to repeat THAT, while in case of BUT we have no need to do that...m i right????

Best Regards
Atul

Hey Atul,

Sorry for the delayed reply here - I was about 1/3 of the way through a response the other day, got called into a meeting, and never got it back...

Quick thing to clarify here before even getting to your questions - the answer is still correct and for very similar reasons, but looking back at it (I'll blame that I was toggling browsers between this thread and the question as an image file on another):

the lack of the word "which" followed by a verb in C actually makes that description just a modifier of "thromboxane", and since it's a valid modifier you can ignore it to just read:

A low dose usually suffices to block production of thromboxane, but does not seriously interfere...

So, really, "does not seriously interfere" in the two choices that don't employ "which is" actually ties back to "a low dose of aspirin", and should therefore be parallel with "suffices to block".


Back to my overall point, though, "which is" isn't wrong for the reason that a lot of people might think. But, technically, it's wrong for another reason: it's (at least I think it is) illogical to say that:

Aspirin suffices to block thromboxane, which is a substance that promotes blood clotting but that does not interfere with another chemical that prevents clogging.

If you use "thromboxane" as the subject for both verb phrases in the second half of the sentence, it's probably illogical - why would the same substance both promote clogging but not interfere with a substance that prevents clogging?

So for that reason, actually, your proposed choice E is probably wrong...although the way we initially talked about it it would have been right (does that make sense?).

________________________________________

As far as "and" vs. "but" and parallelism, it's more a case of the clauses they're linking than those words themselves. For example, you'd say the same thing:

A substance that prevents blood clotting but that also increases risk of blood thinning

and

A substance that prevents blood clotting and that also reduces cholesterol


and the "that" is a pronoun in either case, and not really necessary in the absence of a comma so you could also say:

A substance that prevents blood clotting and reduces cholesterol
A substance that prevents blood clotting but also increases the risk of blood thinning

With the comma, as in your example, you need to reintroduce the pronoun "that" so that there's a subject connected to the verb, and that's the same in either case:

A substance that prevents blood clotting, and one that also reduces cholesterol

Otherwise, if the comma splits the subject from the verb, that is considered a 'comma splice' and therefore incorrect. So you'll need to introduce a subject - typically you'll see them use "that" - there.

I hope that helps...

Hi Brian,

Sorry m replying late, thank you so much for replying but m again going to take some of your time..

In your second post (in reply of my question), i got your point that the VERB coming after BUT logically should have the subject "low dose of Aspirin" ----> completely agree

Aspirin suffices to block thromboxane, which is a substance that promotes blood clotting but that does not interfere with another chemical that prevents clogging.

This u said is wrong because the second THAT in red refer to thromboxane and that makes this sentence illogical.-----> completely agree

Now look at this modified version of Op E:

A low dose of Aspirin suffices to block thromboxane, which is a substance that promotes blood clotting but does not seriously interfere with the production of Prostacyclin, which prevents prevents clotting.

What do you think about this Option: I think this is correct because here verb (in green) is referring to "A low dose of Aspirin"....M i correct here...????? please answer

Thanks

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

The splits:

1. Apposition:

NOUN+ COMMA + Apposition.

Example:

MS Dhoni, the Indian cricket captain, has played well.---Correct.

if we revise the sentence as follows:

MS Dhoni, who is the Indian cricket captain, has played well.---Incorrect.

Explanation: We don't need to use "who is" in the revised option.

Another example:

I have bought a book, which is a collection of poems of Tagore.--Incorrect.
I have bought a book, a collection of poems of Tagore. --correct.

Now, we can easily eliminate the options that have used "NOUN, which is X". A, D, and E are out.

2. In case of two verbs of a subject, we need to use parallel structure:

He likes to help the poor but does not like to help the cheaters.

The option B is not parallel.

The dose suffices but does not interfere.--good parallelism.

Answer is thus C.

Thanks.

@Catseye

1. Apposition:

NOUN+ COMMA + Apposition.

Hi, this point looks good to me...good catch!!