一句英语选择题，求详细解答

Question

All flights ______ because of the terribleweather, they had to go there by train.
A.having been canceled C.having canceled
B.had been canceled D.were canceled

Answer 1

选A；独立主格结构作原因状语。

分析：
首先航班是被取消，所以用被动语态，C排除。

B和D是完整结构，但后半句也是完整结构，两个完整句子必须由并列连词或从属连词连接，这里没有连词，所以B、D排除。

Answer 2

A.having been canceled 是一独立主格作原因状语相当于一从句because all flights had been canceled because of the terrible weather, they had to go there by train.
第二是主动式说明是主语they发出的动作显然不对，所以不选。
第三个，是谓语形式而这句无引导词不能构成从句也就无谓语所以也不选。
最后一个同上。所以只选第一个。