Why does the human heart rate increase when exercising?

Cause you get more exercise and you get the blood all flowing

Your heart speeds up to pump extra food and oxygen to the muscles. Breathing speeds up to get more oxygen and to get rid of more carbon dioxide. When a fit person, such as an athlete, exercises the pulse rate, breathing rate and lactic acid levels rise much less than they do in an unfit person.

The answer is d. when it begins to fall back to the child's hand

Yes yes yes very good