by compacting the topsoil by decreasing the fertility of the topsoil by adding nutrients to the topsoil by removing the minerals from the topsoil
I gave my students this multiple choice question: Earthworms move randomly throughout topsoil. How do earthworms most likely affect the topsoil?

Only one answer is correct among these 4 choices:
- by compacting the topsoil
- by decreasing the fertility of the topsoil
- by adding nutrients to the topsoil
- by removing the minerals from the topsoil

Could you tell me which one is correct?

by adding nutrients to the topsoil