How Does a Crevasse Form

How does a crevasse form?

A) A glacier moves over a volcanic hot spot,causing differential melting and caving it inward.
B) A glacier moves across a ridge of particularly resistant rock,which breaks the glacier from the bottom upward.
C) Internal stress causes a glacier's upper surface to break as the glacier moves.
D) A glacier moves over a pre-existing stream valley,which causes it to fracture on each side of the valley.