第1题: [单项选择]All the flights ______ because of the snowstorm, we had to take the train instead. A. were canceled B. had been canceled C. having canceled D. having been canceled 参考答案:D 答案解析:[考点] 考查独立主格结构。 句子中间使用的是逗号,故该句的前半句应使用独立结构,由于动作发生在过去,所以应该用被动语态的完成时形式,故D为正确答案。