What was the relationship between the Koros and the Mandinkas?

The Koros were a family (clan) that belonged to Mandinka (West African ethnic group). Story about them we find in the novel Roots written by Alex Haley in 1976, which was a story history of his family looking back for a few generations.  

Story begins with Kun-ta who is a member of the Kinte clan.  The Koros and the Kintes are rivals. Koros enslave their own race - they kidnap members of Mandinka and sell them to white people who take them to America to sell them further.  

Koros do that without the consent of their king mostly to trade them for guns.

Africans had slavery even before the Europeans came, but they treated them in a much less cruel way.  

Story goes on to the present time of Alex Haley`s life. It follows his family from Kun-ta and generations after him and their hard struggle under the slavery and racism.

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the correct answer is d