As activity increases, more oxygen is exhaled. As activity increases, breathing becomes more efficient. As activity increases, the oxygen needs of cells increase. As activity increases, the rate of the heart beating increases.
I gave my students this multiple choice question: Ava conducted an experiment to determine respiratory rates during different activities. She counted the number of breaths per minute. Ava noted that the number of breaths increases as physical activity increases. Why is there an increase in the number of breaths per minute as activity increases?

Only one answer is correct among these 4 choices:
- As activity increases, more oxygen is exhaled.
- As activity increases, breathing becomes more efficient.
- As activity increases, the oxygen needs of cells increase.
- As activity increases, the rate of the heart beating increases.

Could you tell me which one is correct?

As activity increases, the oxygen needs of cells increase.