My class teacher didn't notice me in the classroom________he came in．A.becauseB.in order toC.whileD.when
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D解析分析：when 引出时间状语从句，表示“当……时候”的意思，此处不能用while，因为从句中的动词 came 是个短暂动作。(延伸)when引出时间状语从句时，用法上的限制比较少，既可以用于表示先后动作，也可以用于表示同时的动作；既可以用于延续性动词，也可以用于短暂性动词。1)先后动作：Dick got up into a tree as soon as he saw the bear．可以改为Dick got up into a tree when he saw the bear．2)同时动作：He was watching TV when I went to see him yesterday evening．3)延续性动词：I happened to meet an old friend of mine in the street when I was walking back home．4)短暂性动词：I waswalking back home when I saw an old friend of mine．