An insect is food for a salmon. Water carries a rock downstream. A tree removes a gas from the air. A flower makes food for a butterfly.
I gave my students this multiple choice question: Which example shows a relationship between a living thing and a nonliving thing?

Only one answer is correct among these 4 choices:
- An insect is food for a salmon.
- Water carries a rock downstream.
- A tree removes a gas from the air.
- A flower makes food for a butterfly.

Could you tell me which one is correct?

A tree removes a gas from the air.