Which of the Following Is the Best Explanation of Why

Which of the following is the best explanation of why most American Indian reservations are in the West today?

A) Tribes had difficulty crossing the Rocky Mountains when they established their settlements.
B) European settlers and the U.S. government pushed Indian tribes westward.
C) Only the Indian tribes in the West survived after those in the East died of exposure to European disease.
D) Indian tribes moved westward in hopes to expand economic power.
E) Most American Indian reservations are not in the West.