What factor led to the expansion of slave trade?

U.S History

U.S History

U.S History
Govt and TICO sign deal on thermal power expansion
Accra, April 29, GNA – Government and the Takoradi International Company (TICO), a power generation firm, on Thursday signed an agreement for the expansion by 110 mega watts, the thermal generating plant capacity to 330 mega watts.
Read more on Ghanaweb.com

A. was the Muslim leader who monopolized the African gold and slave trade.
B. designed the first caravel, which made long sea voyages possible.
C. discovered the Canary Islands and began the age of exploration.
D. was the Christian prince the Portuguese hoped to find by exploring Africa.

What are the major problems and challenges involved in the study of the history of Bantu expansion in Africa?