Your body needs more oxygen when you are exercising than when you are not exercising. why is this?

This is because the heart is panting to get more oxygen and each breath brings more oxygen in the body and more carbon dioxide is being taken out.

Explanation:

Hope it helps.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.