A:静止
B:向下运动
C:向上运动
D:以上三种情况都有可能
氧气和二氧化碳的交换部位是( )
— What did he _______ just now?— Sorry, I didn't catch his words. I_______ about something else.
显微镜的使用步骤是( )
①取镜与安放
②对光
③安放装片
④调节焦距
⑤观察.