How did the geography of the East African coast give rise to prosperous city states

The city-states along the eastern coast of Africa made ideal centers of trade. As trade intensified between Africa and Asia, prosperous city-states flourished along the eastern coast of Africa. The city-states traded with inland kingdoms like Great Zimbabwe to obtain gold, ivory, and iron. These materials were then sold to places like India, Southeast Asia, and China. These were Africa’s exports in the Indian Ocean Trade. These items could be sold at a profit because they were scarce in Asian countries.