For What Reason Did the Second Wave of Bantu-Speaking Migrants

For what reason did the second wave of Bantu-speaking migrants entering central Africa revert to subsistence farming?

A) The tropical rain forest was so rich in edible plants that humans did not need intensive agriculture.
B) The region was infested with tsetse flies carrying sleeping sickness that wiped out the Bantus's cattle.
C) The Bantus lacked the iron technology needed to farm in that region.
D) The local populations prevented the Bantus from obtaining the best farm land.