Hi ,i see you are giving a lecture again!what's it about well,( ）the chat room ten minutes ago,you would have known what we were talking about!a ;had you joined b;if you joined c;were you to join d;if you should join

Hi ,i see you are giving a lecture again!what's it about well,( ）the chat room ten minutes ago,you would have known what we were talking about!a ;had you joined b;if you joined c;were you to join d;if you should join
Hi ,i see you are giving a lecture again!what's it about well,( ）the chat room ten minutes ago,
you would have known what we were talking about!
a ;had you joined b;if you joined c;were you to join d;if you should join

Hi ,i see you are giving a lecture again!what's it about well,( ）the chat room ten minutes ago,you would have known what we were talking about!a ;had you joined b;if you joined c;were you to join d;if you should join
第二句话的意思是,如果你十分钟之前进聊天室的话就能知道我们在说什么了,这是一个对过去的虚拟条件句,由后面的would have known 可知前面应该用过去完成时,完整的应该是if you had joined ,但省略了if 的话就要把had 提前,所以选a .就这样.多看看关于虚拟语气和情态动词的语法会好一点.