一个英语单选题,我认为选B,可答案给的是A.请教高手到底选什么呢?

题目

一个英语单选题,我认为选B,可答案给的是A.请教高手到底选什么呢?
If you had told us earlier _____ he was, we could have introduced him at the meeting.
A. who B. whom

答案

答案A.
你想：你是谁?用英语怎么说.who are you
who 引导宾语从句.whom不可以 做表语的.
.宾语从句中引导词的用法在复合句中作主句的宾语,引导词有：
连词：that (that 常可省略),whether,if
代词：who,whose,what ,which
副词：when ,where,how,why 等.