How did westward expansion affect Native American tribes?

a
Native American hunters were able to use the railroads to sell bison meat and hides for more money.

b
American settlers killed many bison, greatly lowering the population of this Native American food source.

c
Americans brought enslaved people with them to the west to do farm work. This caused the bison population to increase.

d
Because American settlers created many new farms, Native Americans started eating grain as well as bison meat.