Bradley decides one day to run a marathon without any training. After ten minutes has elapsed and Bradley has run half a mile, he begins to get cramps in his thighs. He is also breathing very rapidly. What is causing the cramps on Bradley's legs? A) The weather must have been too hot to run. B) Too much oxygen was getting to the cells, causing them to cramp. C) The buildup of carbon dioxide caused the muscles to begin cramping. D) Lactic acid buildup in his muscles due to a lack of oxygen to his cells.