When a mammal respires anaerobically for a long time, lactic acid builds up in the muscles, leading to fatigue and cramps. 

• Lactic acid build-up: Anaerobic respiration produces lactic acid as a waste product when glucose is broken down. This lactic acid accumulates in the muscles. 
• Muscle fatigue: A buildup of lactic acid causes muscle fatigue. 
• pH drop: If too much lactic acid builds up, the pH falls, which can affect enzyme function and lead to fatigue and cramps. The correct answer is option C