How Did the Early Europeans Usually Obtain Their Supply of Slaves

How did the early Europeans usually obtain their supply of slaves?

A) They raided along the coast of Africa, forcibly capturing large families.
B) Arabs brought slaves up to the coastal cities in Europe, and they were purchased there.
C) Europeans captured large land areas through warfare, and subjugated entire groups of people into slavery.
D) They obtained their slaves through trade with native African tribes.