根据短文内容，完成下列各题。

    Fire can start in forests, homes and schools. It is very dangerous. What should we do when there is a fire? We should go outside quickly. And we have a fire drill at school every year.

    A fire drill is not a real fire, It helps us prepare for(为……准备) a real fire. This map tells us the way in a fire. We can follow the fire exit(出口) signs too. First, the fire alarm sounds. Next, we all line up and go out quickly. We leave all our things in the classroom. Then we walk down the stairs and go outside. We line up in the playground. Our teacher checks on the number of students. We should wait for the firemen to come.

下列自然现象中，属于凝华的是（　　）

在调查男女乘客是否晕机的情况中，已知男乘客晕机为28人，不会晕机的也是28人，而女乘客晕机为28人，不会晕机的为56人，

其中 为样本容量。