题目内容：

请分析这道题All buses__because of the storm,many passengers chose to take the train.为什么选A呢?
A having been cancelled
B having been cancelling
C had been cancelled
D were cancelled

优质解答

后面才是主句,已经有了主谓,用的是过去式,
那么前半句应该是过去完成时,又不能再出现谓语,所以用ing形式独立主格.
不管是过去完成时还是现在完成时,独立主格时都是having
先A.