阅读理解。 Feel tired lately? Has a doctor said he can't find anything wrong with you? Perhaps he has sent you to a hospital, but all the advanced equipment there shows that there is nothing wrong with you. Then, consider this: you might be in a state of sub-health (亚健康). Sub-health, also called the third state or gray state, is explained as a borderline(界限) state between health and disease. According to the survey by the National Health Organization, over 45 percent of sub-healthy people are middle-aged or elderly. The percentage is even higher among people who work in management positions as well as students around exam-week. Symptoms include a lack of energy, depression, slow reactions, insomnia, anger, and poor memory. Other symptoms include shortness of breath, sweating and aching in the waist and legs. The key to preventing and recovering from sub-health, according to some medical experts, is to form good living habits, alternate work and rest, exercise regularly, and take part in open air activities. As for meals, people are advised to eat less salt and sugar. They should also eat more fresh vegetables, fruits, fish because they are rich in nutritional elements -- vitamins and other elements that are important to the body. Nutrition experts point out that it is not good to eat too much at one meal because it may cause unhealthy changes in the digestive tract (消化道). They also say that a balanced diet is very helpful in avoiding sub-health. 1. According to this passage, which of the following is true? A. When you are in a state of sub-health, you should stay at home and keep silent. B. When you are in a state of sub-health, you should go to see a doctor and buy some medicine. C. When you are in a state of sub-health, you should have yourself examined in foreign countries. D. When you are in a state of sub-health, you should find out the reasons and relax yourself. 2. It may be easy for middle-aged people to get sub-healthy because _________. A. they have used up their energy B. they have lost their living hopes C. they have more pressure in life and work D. they begin to get older 3. The key to preventing you from falling into a state of sub-health is that you should ________. A. work hard B. sleep more C. form good living habits D. take more medicine 4. The underlined word 'alternate' in this passage is closest in meaning to _________. A. arrange by turns B. cause to take place C. keep oneself busy with D. get tired of