She has never been to the Great Wall before.Today she saw it ___ the first time.A.at B.for C.in D.

题目

She has never been to the Great Wall before.Today she saw it ___ the first time.A.at B.for C.in D.
She has never been to the Great Wall before.Today she saw it ___ the first time.
A.at B.for C.in D.by
答案是选B,但我想知道A为什么不对.
还有就是这四个选项中的短语都成立吗,如果成立的话该怎么用呢?请分别说明,

答案