When land has been overfarmed or overgrazed, <span>desertification </span>occurs.
The only logical answer in this question is D. If not than you could have done process of elimination. Animals eat plants and plants initially get energy from the sun. A is illogical because they are not the smallest organism. C isn't right since there aren't more of them. And in B they are never always all eliminated.
We learn about this in my class A.
