Dr. Jones argues that American Indian populations suffered near eradication in colonialists' early explorations of the Americas. Select the two reasons that inform Dr. Jones' argument.

a. The negative influences of colonialists on American Indian populations' ways of living and working perpetuated the spread of diseases.
b. American Indian populations were already struggling to survive, and the nature of the colonial regime worsened their situation.
c. American Indians were biologically different from Europeans.
d. Due to having had no exposure to diseases originating from Europe, American Indians' immune systems were less resistant, accounting for the massive death rate.