Since the 1970's, scientists have believed that an asteroid once

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.

Lots of structural and pronoun reference problems.

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 --- 1. What does which refer to? decade/darkening/ asteroid? 2. 'inhibited and leading' is unparallel

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 -- what does 'it' refer to? same problem as in A? How may ands in the one sentence! Use of 'that' in the second factor and leaving out for the third and the fourth factors are un// and dubious.

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 -- Correct choice

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 -- it is redundant.

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 -- 'a darkening of the sky' does not seem to have a verb