By 1200

c. e. improved agricultural technology had spread throughout much of sub-saharan africa primarily through the (a) development of oxen immune to diseases carried by the tsetse fly (b) discovery of gold that provided a means of exchange among groups (c) expansion of the sahara desert, which forced berber peoples to move south (d) migration of bantu-speaking peoples with their knowledge of ironworking

Options b and d.

Explanation:

Migration of Bantu-speaking people initiated a process of assimilation by these people of the previous communities (more oriented to hunter-gathering) due to their knowledge of ironworking, and their extensive use of cattle. And additionally, this was combined with the discovery of large mineral deposits (gold, but also copper and iron) that would be used for the monetization of the trade routes. This would be the beginning of entities such as the Monomotapa kingdom in South Africa and the Songhai, Ghana and Mali empires.

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