Stella’s respiratory system is not working well. Why is this a problem for her ability to exercise? A) Her respiratory system will not be able to break down molecules from air into oxygen.
B) Her cells will not work well when they have low levels of oxygen.
C) Her digestive system will have to work twice as hard to get the same amount of energy from glucose molecules.
D) Her cells will not work because molecules from air will be too large for her body to use.

In addition to giving an answer, please provide an explanation to support it.