They add oxygen to the soil. They release oxygen into the air. They fix carbon dioxide in the soil. They store carbon dioxide in roots.
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: How are green plants an important part of the carbon dioxide-oxygen cycle?
Possibilities:
- They add oxygen to the soil.
- They release oxygen into the air.
- They fix carbon dioxide in the soil.
- They store carbon dioxide in roots.

They release oxygen into the air.