Why not C?

I have the same question, please explain

C would certainly weaken if it said that volcanic eruptions happened at an increased rate around the same times that the ice ages occurred. However, we don't know this and we have absolutely no reason to think that this is the case. In the absence of a reason to think that eruptions occurred around the same time as the ice ages, we cannot say that this is an alternative cause in this case. However, it could be proof of concept that dust can indeed cause temperatures to fall.