Niagara Falls were formed ________
A) by humans 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
Answer: D
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Answer the following statement true (T) or false (F)