What led to the diffusion of agricultural techniques from the Sahel region to other parts of Africa around 2000 BCE?

the climate became warmer and the desert expanded

Explanation:

The Sahel region is the region of ecoclimatic as well as biogeographic zone of the transition in the Africa between the Sudanian savanna to the south and Sahara to the north.

At around 2000 BCE, the agricultural techniques of the people of the Sahel region started spreading or diffusing to the other parts of the Africa due to the climate change. The climate became more arid and warmer gradually and the deserts also expanded in the region.

d. stage 4

explanation:

the existence of some kind of demographic transition is widely accepted in the social sciences because of the well-established historical correlation linking dropping fertility to social and economic development.[1] scholars debate whether industrialization and higher incomes lead to lower population, or whether lower populations lead to industrialization and higher incomes. scholars also debate to what extent various proposed and sometimes inter-related factors such as higher per capita income,lower mortality, old-age security, and rise of demand for human capital are involved.[2]