All of the Following Theories Have Been Rejected as Explanations

All of the following theories have been rejected as explanations of the triggering mechanism for continental glaciation EXCEPT:

A) plate motions, because they are too slow to account for repeated cycles of glacial advances and retreats.
B) large meteorite impacts, because they are not periodic and their effects are short- lived.
C) volcanism, because it does not occur at regular intervals and its effects are too brief.
D) orbital mechanics, because its effects are too slight to account for global climatic changes.