Who ---- to visit our school last Wednesday?Mr.Smith did.A.d

当who用作动词的主体,用以询问人的姓名、 身分或职位)时,可以用一下表达：
I wonder who phoned this morning.我不知道今天上午谁打来过电话.
Who are the men in white coats?穿着白大衣的那些男的是什麽人?*
Do you know who broke the window?你知道是谁把窗户打破的吗?
当用作动词或介词的宾语时:,可以用一下表达：
Who did you see at church?你做礼拜时看见谁了?*
Who are you phoning?你给谁打电话呢?
而你的这道题比较符合上面一种情况,who是visit的动作执行者,是主语,所以选择B