Please help! What gave rise to the slave trade?

a. The lack of a skilled labor force in the Americas and the decline of the West African population caused residents to seek a living elsewhere.
b. The shipment of gold from the Americas to Europe caused plantation owners to seek inexpensive laborers.
c. The death of many American Indians to disease and the planting of labor-intensive crops.
d. The global expansion of the spice trade and the rise of Christianity in the Americas.