Jon likes hiking. he observes that whenever he hikes up a hill, he breathes heavier than when he is on flat ground. which two statements explain why this happens?

When climbing a hill or mountain, there are different condions which the body reacts to, first: the body begins to exercise, since it must carry all our weight when climbing up a hill, this generates that the heart accelerates and pumps the blood more quickly so that it reaches our whole body out, and the second one, is that at a higher height, less oxygen, if already, our body comes with the heart shaken by the physical exercise, now it has to  accelerate your breathing to get the necessary oxygen for the blood and the body, these two things make us breathe heavily.