What main river runs just west of the appalachian mountains and into the mississippi river?

Which of the following led to a continuation of the treatment in the excerpt above? a)the brutality of the trans-atlantic african slave trade. b)spanish repression after violent native uprisings in new spain during the 16th century. c)european competition to quickly establish colonial holdings in the new world. d)the reliance of the spanish on native americans to provide labor under the encomienda system.