If you had joined the chat room ten minutes ago, you what we were talking about.
A. would know
B. would have known
C. knew
D. had known
网友回答
A. would know
(答案→)B. would have known
C. knew
D. had known
解析：考查虚拟语气。句意“would+have+done”表示对过去事情的假设，意思是“本来会做”。如果你十分钟前加入聊天室，你就会知道我们正在谈论什么了。“would+have+done”表示对过去事情的假设，意思是“本来会做”。故选B。