What Process Resulted in the Formation of the North American

What process resulted in the formation of the North American central lowland that lies in between the two major mountain ranges of the continent?

A) A colossal volcanic eruption split North America in half, and the central lowland, once an intercontinental sea, was filled in by glacial activity.
B) Earthquakes opened and flattened what was once a mountainous area.
C) Material that erodes from the Rocky Mountains has been deposited by wind, rain, and rivers.
D) Early European settlers cleared and flattened a hilly landscape so that they could more easily farm the area.