2016年5月9日19时12分左右，“水星凌日”的天象于本世纪第三次出现．如图所示，水星如一颗小黑痣从太阳“脸上”缓慢爬过．它的形成与下列光现象原理相同的是（　　）

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    Liu Yun, 13, pretty and smart. She is a good student. But many students try to stay away from her because Liu Yun's mum has AIDS(爱滋病) and her father has died of AIDS. Luckily, Liu Yun didn't get it.
    AIDS means fear for most people. Nobody wants to play games with her. Even a teacher who used to like her turns his back on her now. The biggest problem is how other people look at her. She sometimes wants todrop out of schoolbecause she can no longer face others' indifference(冷漠)。
    It is said if a person has AIDS, it's still safe to eat and play with him. So I think Liu Yun's classmates and teachers are afraid of AIDS because they don't know much about AIDS. And Liu Yun needs their love and understanding. Even a friendly smile means a lot to her.

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