If your muscles use up too much ___ during strenuous exercise, they begin producing ___, which is what causes fatigue and burning sensations.
(A) is the correct answer. When your muscles use up too much oxygen, it cannot perform cellular respiration, so instead it reverts to lactic fermentation. The lactic acid that builds up in your muscles then causes fatigue, pain, and burning. Dont confuse it with pyruvic acid, which is the product of glycolysis, made from the splitting of a glucose molecule.