—Have you seen your friend off ?—Yes. I watched the train leaving the station it disappeared from sight.A.unlessB.as ifC.so thatD.until
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D解析试题分析：考查词和词组意思，unless意思“除非”， as if意思“好像”，so that意思“以至于”，until意思“直到……”，句子意思“我看卓火车离开车站直到从我的眼前消失”，所以选D考点：考查词和词组意思
点评：固定词组see off意思“送行”，watch sb do sth意思“看到某人做什么”，watch sb doing sth意思“看到某人正在做某事”，强调动作正在发生。