Niagara Falls Were Formed ________

Niagara Falls were formed ________.

A) by humans as a result of the damming of the St. Lawrence River
B) during Precambrian time approximately a billion years ago
C) so long ago that the river's knickpoint is now gone
D) after the retreat of the Pleistocene ice sheets a few thousand years ago
E) after a series of spectacular earthquakes which are still helping the falls evolve