短文填空。从方框中选择合适的单词填空，使短文通顺、连贯。

Everyone knows that exercise is        . We need to exercise. Doctors say it is good        us. It keeps our bodies strong.
When the daytime comes， we must      This is the time for exercise. Exercise means doing things with the body. There are many ways to      You can walk，run，swim，skate，or play ball games. Make sure you excise in the following ways: you have to        what you’re doing. Exercise      ---but not too much. It is good to exercise     a week. Thirty minutes each time is enough. Try all kinds of things        you find one， two or three sports that feel right for you.
Exercise can be      Friends can exercise   at fitness center， or they can play sports together. How do you exercise?