What was probably the reason for a decrease in the american indian population from 300,000 in 1865 to 240,000 in 1890?

select the best answer from the choices provided.
a. american indians died from disease and hunger on reservations.
b. american indians could no longer hunt bison and provide for their families.
c. american indians died in battles with the u. s. military.
d. all answers are correct.