用显微镜观察口腔上皮细胞临时装片时，若物像模糊不清，可以转动哪个结构使物像更清晰（　　）

    Mary is an American girl. She is now in Beijing with her parents. Mary doesn't know much Chinese, but she is studying it. She often speaks Chinese with her Chinese friends. Sometimes they can't understand her, because she can't speak Chinese very well.

    It's Saturday morning. She goes out. She is walking in the street. She wants to go to the zoo to see the elephants and monkeys, but she doesn't know how to get there. She asks a Chinese boy the way. The boy can't understand her. Then she takes out a pen and a piece of paper. She draws an elephant on it, and shows the picture to the boy. The boy understands, and shows her the way to the zoo.