they are exhaling carbon dioxide. their lung cells are multiplying. their respiratory system stops functioning. they are getting less oxygen to their cells.
I gave my students this multiple choice question: Students run laps around the track in physical education class. After a few laps, the students' breathing gets heavier, and their pace begins to slow. The students are most likely slowing down because

Only one answer is correct among these 4 choices:
- they are exhaling carbon dioxide.
- their lung cells are multiplying.
- their respiratory system stops functioning.
- they are getting less oxygen to their cells.

Could you tell me which one is correct?

they are getting less oxygen to their cells.