Why Did the Second-Wave Migration of Bantu People Turn to Subsistence

Why did the second-wave migration of Bantu people turn to subsistence farming as it traveled southward?

A) The tsetse fly-infested environment did not permit raising livestock.
B) Local populations resisted their migration and pushed them into the desert.
C) The lack of iron tools prevented them from pursuing more intensive agriculture or livestock raising.
D) The rain forests of the Kalahari provided all that they needed,so intensive agriculture was not required.