1.The girl called the police while the alien was buying a souvenir.(改为一般疑问句)
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2.The boy was doing his work when the UFO took off.(对画线部分提问）
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3.You can imagine how strang the alien was.(改为感叹句）
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4.while he was talking with the policeman on the telephone,David saw a dog rushing in.（改为同义句）
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5.I was standing in front of the library when a UFO landed.(对画线部分提问)
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6.I was so busy that it was difficult for me to get out of my seat this morning.(改为两个简单句）
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