the level of carbon dioxide in the blood the level of carbon dioxide in the cells the level of carbon dioxide in the lungs the level of carbon dioxide in the air
I gave my students this multiple choice question: What does the brain monitor to control the rate of breathing during normal activities?

Only one answer is correct among these 4 choices:
- the level of carbon dioxide in the blood
- the level of carbon dioxide in the cells
- the level of carbon dioxide in the lungs
- the level of carbon dioxide in the air

Could you tell me which one is correct?

the level of carbon dioxide in the blood