Plz help me with this

The sahel is located on the edge of the sahara desert. a.north b.west c.south d.east

Prior to the presence of europeans on the continent, the slave trade in africa a) did not exist in any form. b) was limited to internal trade only. c) included the trans-saharan and indian ocean slave trade. d) was rarely needed due to the lack of an agricultural economy.

Who was the first explorer to sail all the way down the mississippi river