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 gave my students this multiple choice question: How are green plants an important part of the carbon dioxide-oxygen cycle?

Only one answer is correct among these 4 choices:
- 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.

Could you tell me which one is correct?

They release oxygen into the air.