Here are three factors that caused slavery to grow in Africa:
Slavery had existed in Africa for hundreds of years, especially in the Middle East. Europeans expanded this already existing slave trade.
Tribal and other wars were a major cause of the growth of slavery in Africa. After these wars, the soldiers from the losing side were taken prisoner and sold into slavery.
Europeans needed cheap labor to work their land in the Americas.
Your correct answer is B, they did not intimidate or enslave Natives. They were at least less selfish than the Spanish, and traded with the Natives, instead of stealing or enslaving.
