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D. Their economies were based on trading gold and salt.

Explanation:

The above option is the correct answer.

The African kingdoms of Ghana, Mali and Songhai were known to have economies that were based on trading gold and salt.

These kingdoms were known to be the greatest western Africa trading states. They dominated the trade of gold, salt and the merchandise that goes between North America and sub-Saharan Africa. These kingdoms were strongly involved in the trans-Saharan trade due to the gold they produced.

They embraced Islam and also established relations with Islamic kingdoms that were neighbors.

answer; i believe the correct answer is (a) the correlation between the cause variable and outcome variable is zero;