Many villages in the sahel have been settled ? a. at the edge of the sahara b. outside of cities c. on the banks of the two main rivers d, near forested areas

Alot of them have settled at the edge of the sahara. So A is probably the best and most accurate answer for you.

980 is the correct answer

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.