Vyshak
Since the 1970's, scientists have believed that an asteroid once landed in the Gulf of Mexico and released ash causing a darkening of the sky by 10 to 20 percent over the ensuing decade, which inhibited photosynthesis and leading to the extinction of countless species on the earth.
A) an asteroid once landed in the Gulf of Mexico and released ash causing a darkening of the sky by 10 to 20 percent over the ensuing decade, which inhibited photosynthesis and leading
B) an asteroid once landed in the Gulf of Mexico and that released ash was what caused a darkening of the sky by 10 to 20 percent over the ensuing decade, and it inhibited photosynthesis and led
C) an asteroid once landed in the Gulf of Mexico and released ash causing a darkening of the sky by 10 to 20 percent over the ensuing decade, inhibiting photosynthesis and leading
D) a darkening of the sky by 10 to 20 percent caused by the released ash over the ensuing decade after an asteroid once landed in the Gulf of Mexico, inhibited photosynthesis and it led
E) a darkening of the sky by 10 to 20 percent caused by the released ash over the ensuing decade after an asteroid once landed in the Gulf of Mexico, inhibiting photosynthesis and it led
Official Explanation
Creating a filter: In this question, the original sentence has an error at the end of the underlined portion: "leading" should be the past tense verb "led" in order to be parallel with "inhibited."
Applying the filter: as we look at the answer choices after (A), we can focus first on the latter two verbs. Choice (B) fixes the problem with "led" but makes other changes; it replaces "which" with a phrase with the pronoun "it," which has ambiguous reference, so (B) is out. Choice (C) has parallel verbs with "inhibiting" and "leading," so it passes our filter. Choice (D) creates an ungrammatical structure in which "inhibited," in coming after a comma splice, has no grammatical verb, so (D) is out. Choice (E) is not parallel between "inhibiting" and "led."
The correct answer is (C).