The Bottom or Downward Tip of a Crevasse Marks the ________

The bottom or downward tip of a crevasse marks the ________.

A) boundary between parts of the glacier moving by basal sliding and crevassal slip
B) boundary between the basal sliding and external flowage zones
C) bottom of the basal sliding zone
D) top of the internal deformation and flowage zone, and the base of the brittle zone of fracture