What would have happen___,as far as the river bank?

题目

What would have happen___,as far as the river bank?
5.What would have happened______,as far as the river bank?（2001上海）
A.Bob had walked farther B.if Bob should walk farther
C.had Bob walked farther D.if Bob walked farther
为什么答案选C,希望得到详细解析.

答案

C.had Bob walked farther
倒装,= if Bob had walked farther
表示与过去事实相反,所以用had walked