What Prevented Many Native Americans from Becoming U

What prevented many Native Americans from becoming U.S. citizens in the nineteenth century?

A) Congress had agreed to keep the long-standing treaty system with Native Americans.
B) The Fourteenth Amendment specifically said they could not be citizens.
C) The Dawes Act did not allow for citizenship.
D) Most Indians were unwilling to cede their tribal setting and assimilate into American society.
E) Service in the Confederacy during the Civil War resulted in blocking citizenship after the war.