Why has some tropical grassland in Africa become desert, like the Sahel?

Answer(say);

Ok so i just did that quiz and here are the answers: 1. how is continental crust different from oceanic crust? a. continental crust is more dense and contains a lot of basalt.b.continental crust is thinner and younger. < c. oceanic crust is thinner and younger.d.oceanic crust is thicker and contains a lot of granite.2. where do subduction zones often occur? a.where a continental plate is being split into two partsb.where an oceanic plate is moving toward a continental plate < c.where two oceanic plates are moving away from each otherd.where two continental plates are colliding3. at which type of boundary do lithospheric plates collide? a.convection boundariesb.contour boundariesc.gravitational boundariesd.convergent boundaries < 4. what is the best description of a convergent boundary? a.plates that move side by side past each otherb.plates that are attracted to each other by gravity  c.plates that move away from each otherd.plates that move toward each other < 5. how can mountain formation be described according to the theory of plate tectonics? a.as two continental plates collide along a convergent boundary, mountains are formed. < b.as a continental plate and an oceanic plate come together at a divergent boundary, mountains are formed.c.as any two plates meet at a fault line boundary, mountains are formed.d. as two continental plates move past each other at a transform boundary, mountains are formed.