Which geographic feature limited the expansion of west african civilization?

a. drakensberg moutains
b. great rift valley
c. nile river
d. sahara desert

All the choices seem possible good answers, but D is the correct one: a. Drakensberg Moutains are the extreme south-east part of the "Great Escarpment", they may result an obstacle only on the far coastal zone (they do lack signs of civilization); b. The Great Rift Valley runs in the mid-east zone, but fossils, even 3 million years old, were discovered here, a proof of civilization; c. The Nile River was actually a resourceful water bank for the flourishment of the Egyptian Empire, it wasn't an obstacle for West African expansion per se; D. The Sahara Desert is the main and greatest geographical feature (among these choices) stopping people from West civilized expansion, in fact, only nomads have lived and still live there.