Tricky CR : weaken question

[RonPurewal]

Whereas C merely says that the temperatures were higher than expected, which means atmosphere could be slightly less cooler than expected.

the hypothesis is that eruptions cause the atmosphere to become "cooler than it would be otherwise".

choice C describes an instance in which the atmosphere was "higher than expected, given the long-term weather trends".
this is equivalent to saying "...higher than it would be otherwise".

[RonPurewal]

most importantly—
DO NOT question the official correct answers!

here's a handy FAQ:

Q: Is GMAC's correct answer correct?
A: Yes.

Q: Is GMAC's correct answer wrong?
A: No.

Q: I disagree with the correct answer. Who's wrong, me or GMAC?
A: You.

Q: I wrote this other version. Is it better than GMAC's correct answer?
A: No.

I think A is more plausible than C.

this ^^ is not the right way to ask your question.

rather, you should ask in the same way as akhilesh above: "Why is MY reasoning WRONG?" ...because that's the only way you can improve.