The african sahel probably did not become a corridor of communication comparable to the eurasian steppe because of its

Here were the choices on that question: 
a. poverty
b. lack of adequate rivers
c. mountains
d. richness

The answer is D. richness.  Based on A Companion to World History, the African Sahel can be a corridor of communication between Nile and Niger valleys but did not materialize. The reason was the richness of Sahel that it that it can provide for itself even without trade. 

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