If you had joined the chat room ten minutes ago, you what we were talking about!
A. have known
B. knew
C. would have known
D. would know
网友回答
A. have known
B. knew
(答案→)C. would have known
D. would know
解析：考察虚拟语气的用法。根据If you had joined the chat room可知，该句是表示对过去的虚拟，主句选用would have done结构
句意为，如果你早十分钟进入聊天室的话，你就会知道我吗在讨论什么了。