"The Significance of the Frontier in American History" Lecture Concluded

"The Significance of the Frontier in American History" lecture concluded what about the West?

A) The movement westward acted as a safety valve that counteracted the threat of social unrest.
B) The native peoples deserved the land more than the whites.
C) Immigrant labor needed to be recruited heavily.
D) The farmland was too arid, so most states and territories needed to focus on manufacturing.
E) The region was too large geographically, so not enough railroad track would ever be built to connect the region.