Answer:
C. After running 20 miles
Explanation:
Anaerobic respiration only occurs when an organism lacks all the oxygen we need to function. After running 20 miles, you will be out of breath, and you will have been using more energy than your respiratory system could produce alone. Due to this, your body will start using some of your energy stored within cells in order to continue functioning. This kind of respiration is basically 'burning calories'.