They are replenished by sunlight. They are cycled through ecosystems. They are replaced by volcanic eruptions. They are produced constantly from nutrients.
I gave my students this multiple choice question: Which of the following explains why elements, such as carbon and oxygen, that are used in organic molecules are not permanently removed from the environment?

Only one answer is correct among these 4 choices:
- They are replenished by sunlight.
- They are cycled through ecosystems.
- They are replaced by volcanic eruptions.
- They are produced constantly from nutrients.

Could you tell me which one is correct?

They are cycled through ecosystems.