Which Statement Best Summarizes the "Frontier Thesis

Which statement best summarizes the "frontier thesis"?

A) Continuous expansion set new standards for democracy, strengthened individual character, and fostered a collective identity.
B) As the "free" land in the West was depleted, Americans were forced to embrace technology, and thus continuous expansion fostered the growth of American business.
C) Continuous western settlement tested the standards for democracy, although it helped develop the rugged individualism that is part of the collective American identity.
D) The depletion of available land in the West brought to an end the period in U.S. history that emphasized individualism and strength of character over democracy.