During the day we work and play, and at night we sleep. Our bodies rest while we are 1. In the morning we are ready2 again. Our bodies grow 3 while we are asleep. Children who are tired usually 4 sleep. We can get better at our lessons 5 we have had plenty of rest. Boys and girls who are eight or nine years old need ten hours of sleep every night. Our bodies need 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 lungs(肺) need 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.

加入5%柠檬酸钠的血液静置一会儿后，中间一层白色物质是（  ）

下列说法正确的一项是（　　）