选出能完成句子的最佳答案。
(B)
Everyone knows that exercise is important．We all need to exercise．Doctors say it is good for us．It makes your heart and body strong．Children who often exercise are more alert(灵活的).That means they do better in tests and schoolwork than those who don't exercise．
There are many ways to exercise．You can walk，run，swim，skate or play ball games．Make sure you exercise in the following ways：You have to like what you are doing．Exercise enough--but not too much．It's best to exercise twice a week．Thirty minutes each time is enough．Try all kinds of sports until you find one，two or three sports that feel right for you．
You can exercise at fitness centers．They have a lot of equipment(设备)there．The equipment will help exercise your arm，legs and other parts of your body to make you healthy．Some people buy equipment for their homes．But it's very expensive．Exercising can be fun．Friends can exercise together at fitness center，or they can play sports together．How do you exercise?
