求详解几道英语单选题 谢谢

题目

求详解几道英语单选题 谢谢
我手头有正确答案
重要的是 请帮忙的朋友附上解释 感激不尽!
1.several had passed___the puzzled travelers came to understand what was going on.
A. after B. when C. before D. until
2. ___is always the case, the darkest hour comes before the dawn.
A. That B. It C.As D. What
3. Bob said proudly that he had achieved all his goals, not in spite of his handicap, but ___it.
A. owing to B. due to C. because of D. on account of
3. A. owing to B. due to C. because of
这道题选的Because of 。
不解的是 这几个不是都表示“因为”吗？ 为什么只能选C？

答案

第一题,前面是had过去完成时,后面是came过去时,所以连词用before,表示pass的动作发生在came之前,过去的过去
第二题,逗号前后两个句子的成分都很完整,由于没有连词,所以不能用主语,只能用as
第三题,owing to表示由于的时候一般是否定的,不好的结果,due to 单纯指因为某事而发生另一件事,because of后接名词词组,表示因为,由于,这个较常用,剩下两个虽然有时候可以互用
不知道你明白了么?我是从老师那儿听到的,表达不当之处请原谅