Small animals, such as mice, have more active mitochondria in their bodies, working at a faster pace to produce energy and heat. This also translates into a faster heartbeat and breathing rate. Large animals like elephants have a slower metabolism and more chilled out mitochondria, producing just enough heat to live.
You shouldn't over continue to play even though it still hurts which gets B out of the way. You don't need to stop playing tennis completely because when you can get better you can play so it's not D. And only playing once a week could still cause injuries.
