Why did the east african city-states become important trading centers?

The city-states along the eastern coast of Africa made ideal centers of trade. An important attraction was the gold obtained from inland kingdoms. The gold was needed mainly for coins, although it was also used for works of art, ornamentation on buildings, and jewelry.

answer; i believe the correct answer is (favorable balance);

it foreshadows the theme of the paragraph and hints the topic that will be discussed in the paragraph!