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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: Planting legumes in order to add nitrogen to soil most likely causes
Possibilities:
- 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.

an enrichment in the fertility of the soil.