Concept:Muscle fatigue is caused by lactic acid buildup during anaerobic respiration in strenuous exercise.Explanation:During heavy exercise, oxygen supply to muscles becomes insufficient.Muscle cells then respire anaerobically, producing lactic acid.This lactic acid accumulates in the muscles, lowering the pH of muscle tissue.Increased acidity interferes with muscle contraction and leads to fatigue.Answer:C. accumulation of lactic acid