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Dhwanii
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"Which" and "substance" both function as noun modifiers here, so if there were ambiguity about what "substance" were referring to, you'd have the same ambiguity for "which."

And context makes it pretty clear that both must be describing "thromboxane." If we don't know what thromboxane does, it's not going to be terribly meaningful that aspirin blocks it, right? And "thromboxane" also happens to be the closest noun to the modifier. Certainly not a rule, but if a noun modifier is right next to a noun it could logically describe, it isn't wrong.

I hope that clears things up!

Thanks for your reply GMATNinja sir, if my understanding of noun modifiers is correct usage of 'which' won't have the ambiguity issue posed by 'substance' (a flexible noun modifier) because 'which' is constrained by touch rule while flexible noun modifier 'substance' can modify farther away noun. 'Substance' can modify thermoboxane or aspirin (though illogical, it still can, thus creating ambiguity)
The "touch rule" isn't actually a rule, and there are plenty of exceptions to it. (For more on that, check out "Usage #4" in this article.)

So, no, I wouldn't say that the "which" is somehow less ambiguous. Tying "substance" back to "aspirin" would be just as much of a stretch as tying "which" back to "aspirin," so that isn't a significant decision point in this case.
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Hi Experts,

I realize that the use of the comma between "clotting" and "but" is discussed on this form, but I am still a bit confused.

In Choice C: "a substance that promotes blood clotting, but does not seriously interfere with the production of prostacyclin..."

To me, if you are using ",but" shouldn't there be an "it" to refer to "a substance"? Why is the comma absolutely necessary before "but"? It seems to read fine without it: "a substance that promotes blood clotting but does not seriously interfere with the production of prostacyclin..."

Thank you!
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woohoo921

The "but" part belongs to the main clause: "A low dose of aspirin suffices to block the production of T, but does not interfere with the production of P."

If we wanted "but does not interfere" to continue the modifier for T ("a substance that promotes X but does not interfere with Y"), we'd want to leave the comma out to make that intention clear. As it is, the presence of a comma after "clotting" in all 5 choices is a strong indicator that the author does NOT intend the following portion to be read as part of the modifier.
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