Mary is a long-distance runner and trains regularly. Recently, Mary and her husband have unsuccessfully tried to conceive a child. To improve her chances of getting pregnant, why does her doctor suggest that she should stop training?

A) Low levels of body fat may lengthen the menstruation cycle.
B) Low levels of body fat may cause menstruation to cease.
C) Low levels of body fat may lead to excessive levels of estrogen and progesterone.
D) Low levels of body fat may increase fat deposition, thereby decreasing fertility.