Last Saturday, we were walking along the street a car suddenly knocked an old man over.A

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解析：本题着重考察when与while和as的区别。when可以和延续性动词连用，也可以和短暂性动词连用；而while和as只能和延续性动词连用。 如：Why do you want a new job when you have got such a good one already？（get为短暂性动词,不可用while或as）你已经找到如此好的工作，为何还想再找新的？ 本题从句中knock为短暂性动词，故选A正确。