Soil vs earth
 

In the following exercise, I'm supposed to choose the correct option:
"What do scientists mean by soil/earth erosion and what causes it?"
The answer key gives "soil" as the correct answer, but I seem to be having some problems to see the difference between these two, particularly in this context, so any help would be greatly appreciated. Thank you.

Re: Soil vs earth
 

My 2 cents:

I think here they are using "earth" in the whole sense. The whole earth is not eroding. Generally when one using earth and erosion together, you would say something like"erosion of earth's surface" which is .. the soil.