Jimmy is running laps during his physical education class. As he is running, he starts to breathe harder as
his body tries to get more oxygen. Which of the following best explains why Jimmy's oxygen requirement
increases?
A. His cells use oxygen as their primary source of energy.
B. His cells need oxygen to help break down sugars for energy.
C. His cells need oxygen to increase the levels of sugars in the blood.
D. His cells use oxygen to transport substances across their cell membranes.