What started the runaway effect that led to the Sahara area of North Africa becoming a des

What started the runaway effect that led to the Sahara area of North Africa becoming a desert？

A．The prevailing winds became stronger.

B．The seasonal rains moved to a different area.

C．The vegetation started to die off in large areas.

D．The soil lost its ability to retain rainwater.

此题为多项选择题。请帮忙给出正确答案和分析，谢谢！