How did westward expansion affect Native American tribes?
a) Railroad expansion allowed Native American hunters to sell bison meat and hides at higher rates than before.
b) Railroad expansion brought U.S. settlers in contact with bison, drastically reducing the population of this food source.
c) Westward migration of U.S. settlers increased the number of farms, causing a supplementation of bison meat with grains.
d) Westward migration of slave-owning settlers diminished the demand for bison as a beast of burden, causing the bison population to increase.