remove earthworms add more water to the soil increase the amount of pesticide he uses mix decomposed plant matter into the soil
I gave my students this multiple choice question: Drake has started a garden. What can Drake do to enrich the soil so that his plants will grow better?

Only one answer is correct among these 4 choices:
- remove earthworms
- add more water to the soil
- increase the amount of pesticide he uses
- mix decomposed plant matter into the soil

Could you tell me which one is correct?

mix decomposed plant matter into the soil