肺泡中的氧气要进入血液，需要穿过几层细胞（　　）

图示 表示一张纸片被图钉固定在墙上，可以绕图钉旋转这张纸片。下面(    )图是纸片绕图钉旋转后得到的。

根据短文内容，用方框中所给单词的适当形式填空，并将答案写在相应的横线上。每词限用一次。

    Hi, I'm Laura. Today is Sunday. My family are at home. What are they doing now? Let me tell you. My ____ are watching a movie. They like CCTV News very much. My father is reading a ____ about sports. My mother is ____ soup in the kitchen. Steve is ____ brother. Look! He is ____ English on the computer.

动物的先天性行为的控制因素是（　　）

阅读下面漫画。漫画给我们的启示是       。