A marathoner's leg muscles often feel heavy and prone to cramping after 20 or more miles. Which of the following is the best explanation for this?
O A marathoner's circulatory and respiratory system cannot take in sufficient oxygen, and the
muscle cells must switch to anaerobic respiration for energy.
O A marathoner's muscle cells contract when glucose reacts with oxygen.
O A marathoner's circulatory system takes in too much oxygen during a race, which causes
muscle cramping.
O A marathoner's muscle cells cannot store glucose for use as energy.