For a science project, Brad takes his temperature several times a day over the course of a week. He only takes his temperature when he is relaxed and has not been exercising. When he charts his temperature vs the time of day, he gets the following graph: The graph shows that Brad's body temperature A. is affected by many different variables. B. is a cycle that is 24 hours long. C. controls his patterns of sleeping and being awake. D. changes by 4°C over the course of a day.