The Chapter Introduction Tells the Story of the Sioux Migration

The chapter introduction tells the story of the Sioux migration to the Great Plains to make the point that

A) the American ideology of Manifest Destiny meant manifest destruction for tribal cultures.
B) Mexican advancement northward, as well as Anglo-American advancement westward, put pressure on the Plains tribes.
C) frontiers were multidimensional and mobile, involved a variety of peoples and cultures, and ultimately proved as disruptive to the settled East as to the contested West.
D) the United States not only had to resort to war and diplomacy to expand its borders, it also had to cope within its borders with native peoples who attacked the emigrants on the Overland Trail.