题目内容
17．Tom ______ an experiment when someone knocked at the door．（ ）| A． | does | B． | did | C． | is doing | D． | was doing |
分析 当有人敲门的时候，Tom正在做实验．
解答 答案：D．
根据when引导的时间状语从句用了一般过去时，判断设空处的动作在过去某个时刻，即"有人敲门的时候"正在进行着，且动作在一段时间内持续进行，所以用过去进行时，过去进行时的构成是：主语+was/were+动词的现在分词，主语是Tom，所以be动词用was，故选D．
点评 本题考查了动词的时态．做题时，一定要特别关注有关的时间短语，这是判断句子时态的标志．在没有时间状语的情况下可以联系上下文根据语境来做出正确判断．
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