阅读下列材料，根据阅读内容选择正确答案，并将答案写在题号前。 During the day we work and play, and at night we sleep. Our bodies rest while we sleep. In the morning we are ready to work and play again. Our bodies grow most while we are sleep. Children usually need sleep when they are tired. We can get better at our lessons after we have had plenty of rest. Boys and girls need ten hours of sleep every night when they are eight or nine years old. Our bodies need plenty of air when we sleep. If we don’t get enough fresh air, we will be tired when we wake up. While in bed we must not cover our heads. Our lungs need to get enough fresh air. If we open our windows at night, we can have plenty of fresh air. Cool air is better than warm air. Boys and girls must get plenty of sleep if they want to be strong.( ) 16. Our bodies grow most while we are _____. A. eating B. playing C. sleeping D. exercising( ) 17. Too little sleep makes us _____. A. tired B. hungry C. happy D. grow( ) 18. ____ may cause us to feel tired in the morning. A. Too much air B. Not enough fresh air C. Too much cold D. Too much sleep( ) 19. How many hours of sleep should 9-year-old children have? A. 8 hours B. 9 hours C. 10 hours D. 11 hours( ) 20. What does the lung need most? A. Fresh air B. Cool air C. Warm air D. Exercise