If you had joined the chat room ten minutes ago, you what we were talking about.
A. will know
B. are knowing
C. can know
D. would have known
网友回答
A. will know
B. are knowing
C. can know
(答案→)D. would have known
解析：考查虚拟语气。根据语境考查与过去事实相反的虚拟语气，故主句用would have done。句意：如果你一分钟之前加入聊天室的话，你就会知道我们在谈论什么的。