What Feature Helped Integrate Eastern and Southern Africa into Long-Distance

What feature helped integrate eastern and southern Africa into long-distance trading systems?

A) Monsoon winds made east Africa a logical end point for Indian Ocean trade routes.
B) Mandinka people were the most active traders in the Indian Ocean and established entrepôt cities along the coast.
C) Iron mines in the area were exploited by Bantu-speaking people during their migrations.
D) Timbuktu and Jenne were major trade centers.