Answer: B. Proximity to water sources needed for subsistence farming
Basically, Sub-Saharan Africa is a continent of small landowners and its environments are very sensitive. Even with low population densities the soil starts to degrade and future production is threatened by the use of traditional methods.
With the exception of humid zones, climatic realities result in constant crop failures because of drought or dry spells in the growing season. The composition of most African soils consists of highly weathered granite and gneiss parent material, and since the capacity of African soils to hold water is poor, rainfall infiltrates soluble nutrients.
Finally, once vegetation is removed, the erosive action of water and wind eliminates the top soil and crusts appear in the surface making it sealed against infiltration.