[单项选择]During the day we work and play, and at night we sleep. Our body rests while we are asleep. In the morning we are ready to work and play again. Our body grows most while we are asleep. Children who are tired usually need sleep. We can get better at our lessons after we 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 air when we sleep. While in bed we must not cover our head. Our lung needs to get enough fresh air. If we open our windows at night, we can have plenty of fresh air. Boys and girls who want to be strong must get plenty of sleep.

Our body grows most while we are
A. eating
B. playing
C. sleeping