Prep test 31 section 3 - Necessary assumption - Asteroids and extinction of dinosaurs

The answers to this question are as follows:

A) any collision of an asteroid with the Earth would have occurred on a land area rather than an ocean

B) dinosaurs in the neighborhood of an asteroid impact but not within the zone of direct impact would have survived such an impact

C) any event that takes place over a long period of time has many different causes

D) dust from the impact of an asteroid on the Earth would not have had any cooling effect on the climate

E) no more than one large asteroid struck the Earth during the period when the dinosaurs were becoming extinct

Why in AC "D" do we not get to make the assumption that the cooling effect on the climate could lead to the extinction of dinosaurs, but in AC "E" (the correct answer) we are supposed to make the assumption that more asteroid impacts could lead to the requisite amount of dust, rendering the dinosaurs extinct?

Any help on this would be great. Thanks.

I've seen lots of debate over whether or not some statements have the ability to strengthen or weaken arguments. The general consensus seems to be that they cannot strengthen or weaken, however, I came across prep test 29, section 1, question 16 on Proto-Indo-European languages where the correct answer to this weakening question was answer choice B, which was a some statement. I'm not sure if we can post questions in their entirety so I'll just say what the answer choice states and hope anyone with this full question and some good insight can reply "Some languages lack words for prominent elements of the environments of their speakers." My analysis is that while this may be true why does it have to apply to the group referred to in the stimulus? It could just as easily not apply to the group referred to in the stimulus, in which case it would not weaken the argument. Why should I have to make the assumption that the answer choice applies to the group referred to in the stimulus? If anyone can clarify for me it would be greatly appreciated. Thanks.