If you often feel tired and the doctor can't find anything wrong with you, you may be in a state of subhealth(亚健康). Subhealth is a state between health and illness. Most of the subhealthy people are middle-aged ones who are usually stressed out because of work and family. And some of them are students who are having exams.
    If you are subhealthy, it is not difficult for you to become well soon. You should have good living habits. For example, you should get up early and go to bed early. And you should exercise regularly（有规律地）. Exercising can keep you healthy. As for meals, it is better to eat less salt and sugar. Vegetables, fruit and fish are important to your body. You should eat more of them. It is not good to eat too much at one meal, because it may cause unhealthy changes in the digestive tract（消化道）. And at last, a balanced diet is very helpful in staying away from（远离）subhealth.

掉在水平地面上的弹性小球会跳起，而且弹跳的高度会越来越低。下图所示是小球弹跳的频闪照片，小球在1、2位置的高度一样。下面说法正确的是(  )