Choose the best answer which can plete the sentence.I ________ when the doorbell rang.A.was happened to go outB.happened to be going outC.happened to go outD.happened to have gone out
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B解析本题涉及到了happen的用法．大体上说happen表示“碰巧”的时候有三个用法：后可接to do表示一般时；to be doing表示进行时和to have done表示完成时．根据题干的含义“当门铃响的时候，我碰巧正要出去”，应用进行时，因此