Answer the following question given this paragraph: 

A: You can tell that they are different compounds from their very different properties. Carbon dioxide is a harmless gas that living things add to the atmosphere during respiration. Carbon monoxide is a deadly gas that can quickly kill people if it becomes too concentrated in the air.


Q: What type of gas is a harmless gas that living things add to the atmosphere during respiration?

 Choices:

- helium

- oxygen

- Hydrogen

- carbon dioxide

A:
carbon dioxide
