For Reasons That Are Uncertain, Hibernating Animals Periodically Rewarm and Recool

For reasons that are uncertain, hibernating animals periodically rewarm and recool during the hibernation period. It is thought that this is because

A) they need to void wastes.
B) they need to eat a little to restore glycogen stores, especially in their brains.
C) they need to rewarm to activate their immune systems in case they've become infected.
D) two of these.
E) all of these.