Muscle cells in the human body can convert glucose into energy during moderate exercise. During intense exercise, however, the muscles become tired, lactic acid is produced, and pain can set in. Which statement best explains how muscle cell processes change as moderate exercise becomes more intense?

A. Oxygen becomes less available for respiration.

B. Carbon dioxide becomes more available for respiration.

C. Glucose is completely depleted during respiration.

D. ATP becomes depleted during respiration.