What Was the Primary Means by Which European Slave Traders

What was the primary means by which European slave traders obtained African slaves?

A) They bartered for slaves from African slave traders.
B) They launched raids on poorly defended African villages.
C) They conquered large swaths of African territory and enslaved the people within their new colonies.
D) They used their superior military technology to force African rulers to provide them with slaves from the rulers' own kingdoms.