英语高手们~帮我解道高三英语题~

英语高手们~帮我解道高三英语题~
~All flights ____because of the terrible weather,they had to go there by train.
A having been cancled B had been cancled
C having cancled D were cancled
--我需要详细解答啊～谢谢拉～
我也选B的~答案是A~是有关独立主格的~这个我一点都不懂哎~