In Post-Civil War America, Plains Indians Surrendered Their Lands ONLY

In post-Civil War America, Plains Indians surrendered their lands ONLY when they

A) chose to migrate farther west.
B) received solemn promises from the government that they would be left alone and provided with supplies on the remaining land.
C) lost their mobility as the whites killed their horses.
D) were allowed to control the supply of food and other staples to the reservations.
E) were defeated militarily by the U.S. Army in various Indian wars.