After several miles of running and sprinting in intervals, the runner’s muscles ache and feel fatigued. which is the best explanation for what has occurred?
a. the runner's muscles have accumulated lactic acid.
b. the runner’s body has failed to perform cellular respiration.
c. the runner’s cells are relying completely on aerobic respiration.
d. the runner’s muscles have accumulated carbon dioxide.

The correct answer is  (A) the runner's muscles have accumulated lactic acid.

When the muscles of the body performs cellular respiration in the pressence of oxygen the end product is carbon dioxide and water. When the body performs exercise, running, they feel fatigue and muscle ache.This is because anaerobic respiration takes place in the muscles and the end product of anaerobic respiration is lactic acid formation. This acid accumulates in the muscle cell and causes fatigue and muscle ache.

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.