Your heart normally beats 70 to 80 times a minute, but the rate changes depending on your activity level. When you exercise, your body needs more oxygen than when you remain still. Your heart automatically responds to these changing needs. When you need more oxygen, your heart beats faster to increase the flow of blood to your body.

Which of the following best summarizes the passage above?
A.
Your body needs a lot of oxygen when you exercise. That is why you breathe heavier when you are running or shooting hoops with friends.
B.
Your heart rate is dependent on your level of activity. Your heart beats faster when you are exercising and slower when you are still.
C.
Your heart usually beats about 70 to 80 times in one minute. That is more than one heart beat for each second. That is pretty fast!
D.
Your heart makes blood flow throughout your body. It also allows oxygen to move into your muscles, lungs, and even into your bloodstream.