According to many first hand accounts, why were africans sold into slavery and shipped to the americas?

a
european slavers arrived with the specific purpose to steal africans for slave labor in the colonies.

b
african kingdoms such as songhai and benin sold their captive enemies into slavery.

c
europeans traded native american laborers for african laborers with african kings.

d
african slaves were sent to the middle east and traded with europeans for goods, then shipped to the americas to process raw materials.