What role did Americans' belief in Manifest Destiny play in the "closing" of the Western frontier?
A. It was used to justify the settlement of all the land
between the east and west coasts.
B. It helped them rationalize America's tendency to avoid
treaties with neighboring nations.
C. It explained why the West was so difficult to govern
and provided a solution to lawlessness.
D. It supported their aggression toward Native
Americans, who were seen as threats to security.