they are exhaling carbon dioxide. their lung cells are multiplying. their respiratory system stops functioning. they are getting less oxygen to their cells.
I am hesitating between 4 options to answer the following question, which option should I choose?
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
Possibilities:
- they are exhaling carbon dioxide.
- their lung cells are multiplying.
- their respiratory system stops functioning.
- they are getting less oxygen to their cells.

they are getting less oxygen to their cells.