Answer:
B. The conditions in their muscles have become anaerobic.
Explanation:
In humans, cellular respiration occurs naturally in an aerobic condition (presence of oxygen) but anaerobic respiration can be adopted in specific or extreme cases. One of those cases is after or during a VIGOROUS EXERCISE.
What happens is that, after a vigorous exercise by an athlete, oxygen becomes insufficient in their muscle cells due to high consumption. Hence, their muscle cells switch to anaerobic respiration (lactic acid fermentation), which causes the build up of lactate in the muscle tissues. This lactate build up cause athletes to experience a painful and burning sensation in their muscles.
Hence, it all begins when "conditions in their muscles become anaerobic after an exercise".
