All 4 contributed, but B and D are the most direct causes.
I believe it was slaves. From what I know, slaves were widespread all over Africa. I hope I helped.
The correct answer is letter A. Rome was located on a large river that is why they built aqueducts and sewage systems that eventually paved the way for their great civilization. Water is the main source of agriculture, building, and health for the Romans. Today, a lot of the innovative machines we have are influenced by the Roman Empire.
C. Believed immigrants from certain places of origin could not absorb American values.