问一道四级英语选择题All the flights( )because of the snowstorm,we had

正确答案是D.
1.关键：前后之间用逗号隔开,但是却没有用连词连接.
这说明这不是由两个独立句子构成的并列句,而是单个句子.
而且逗号前的结构其主语all the flights和句子主语we又不同,所以前面是个独立主格结构,它在句中做（原因）状语.
2.独立主格结构的构成：独立主格结构,句子.
详细一点可写成：（非谓语动词的）主语+非谓语动词形式,句子.
也就是说all the flights后面不能用动词的谓语形式,而是非谓语形式.
如：His father being ill,he had to stay at home.
3.独立主格结构语法较为复杂,楼主请参看我的另一回答：
4.之所以不选B,就是因为all the flights had been canceled because of the snowstorm已经是个完整的独立句子,而不是独立主格结构了.
即had been canceled在句中做了谓语,而不是非谓语形式.
除非改写成下列句子才是正确的：
1）All the flights had been canceled because of the snowstorm,and/so we had to take the train instead.
或2）Because all the flights had been canceled because of the snowstorm,we had to take the train instead.
5.A were canceled 也同B一样,构成两个独立句,错误!
C having canceled 虽然是非谓语结构,但是用主动形式,错误!