If a person is breathing normally, which of the following could prevent him from using his body to exercise normally?
a) not getting enough sunlight, because cells need sunlight to function
b) not getting enough sleep, because the body only makes energy for exercise while asleep
c) not eating enough food, because cells need molecules from both food and air to work
d) nothing, because he would be able to exercise normally as long as his cells are getting molecules from the air