_______yesterday,you would have met Professor Jones.But now

B.Had you come
If you had come yesterday,you would have met Professor Jones.But now he has left for London.
虚拟语气的用法,表示与过去事实相反
原句的意思是如果你昨天来就好了,你就可以遇到Jones教授了.但是现在他已经离开到伦敦了.
这是一个对过去时态的虚拟,与过去事实相反,结构应该是 if +主语+had +过去分词,主语+would（should could might）+have +过去分词,也可以省略if,用had +主语+过去分词.

这个问题的答案。。
B. Had you come

B
Had you come=If you had come yesterday,.......
因为有个yesterday,所以表示过去的虚拟
从句用过去完成时，主句用过去将来完成时 would have met