Answer:
b. many Native Americans had died from disease and they needed "replacement workers"
Step-by-step explanation:
This would be the right option as Native Americans were initially used as slaves by the Europeans, but many of them suffered from the diseases brought by the Europeans.
The Africans were stronger and could fight these diseases a lot better. They started the slave trade with the Africans, and the Africans proved to be better workers, and better replacements.
Overall: Africans were brought in as replacements due to diseases greatly affecting the Native Americans.