Everyone knows that exercise is important. We all need to exercise. There are many ways to exercise. You can walk, run, swim, skate, or play ball games, but you should remember to exercise in the right ways.

        ①    Before you do any exercise, make sure to warm up for at least five minutes. During warming up, you should extend all parts of your body.

        ②      It's best to exercise twice each week. Thirty minutes each time is enough. Try all kinds of things until you find one, two or even three sports that feel right for you.

        ③    There are a lot of equipments (设备) at fitness centers. The equipments will help exercise your arms, legs and other parts of your body to make you fit. Some people buy equipments for their homes. But it is very expensive.

        ④     If you get hurt when you are doing exercise, don't play again until you are well.

如图是模拟膈肌运动的实验装置及实验过程．请回答