根据短文理解, 从A、B、C三个选项中选择最佳答案填空。

    During the day we work and play, and at night we sleep. Our body rests while we are  1. In the morning we are ready 2 again. Our body grows 3 while we are asleep. Children who are tired usually 4 sleep. We can get better at our lessons 5we have had plenty of rest. Boys and girls who are eight or nine years old need ten hours of sleep every night. Our body needs plenty of 6 when we sleep. If we do not get enough fresh air, we 7 tired when we wake up. While in bed we must not cover our 8, our lung(肺) needs to get enough fresh air. If we 9 our windows at night, we can have plenty of fresh air. Cool air is better than warm air. Boys and girls who want to be 10 must get plenty of sleep.