a decrease in the amount of acid rain. an enrichment in the fertility of the soil. an increase in the ratio of nitrogen to oxygen in the system. a reduction in the soil's ability to manufacture its own nitrogen.
I gave my students this multiple choice question: Planting legumes in order to add nitrogen to soil most likely causes

Only one answer is correct among these 4 choices:
- a decrease in the amount of acid rain.
- an enrichment in the fertility of the soil.
- an increase in the ratio of nitrogen to oxygen in the system.
- a reduction in the soil's ability to manufacture its own nitrogen.

Could you tell me which one is correct?

an enrichment in the fertility of the soil.