问题补充：
(·青岛一模)All flights owing to the heavy fog, many passengers could do nothing but take trains.
A. to be cancelled
B. were cancelled
C. have been cancelled
D. having been cancelled
答案：
A. to be cancelled
B. were cancelled
C. have been cancelled
(答案→)D. having been cancelled
解析：选D考查独立主格结构。逗号前后两部分之间没有连词，因而前半部分不可能是句子，排除B、C两项。此处只能用独立主格结构，动词不定式在此结构中表示将来，故排除A项。现在分词的完成被动式表示被动、完成，故选D项。
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