问题描述：
英语翻译
The boy was walking down the street when the UFO landed.
While the boy was walking down the street,the UFO landed.
The girl was shopping when the alien got out.
While the girl was shopping,the alien got out.
The boy was walking down the street when the UFO landed.
While the boy was walking down the street,the UFO landed.
The girl was shopping when the alien got out.
While the girl was shopping,the alien got out.
问题解答：
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