Unless to speak you should remain silent at the conference.A invitedB invitingC being in

问题补充：

Unless to speak, you should remain silent at the conference.

A invitedB invitingC being invitedD having invited

答案：

解析：本题考查动词的-ed形式。You 与invite 之间构成被动关系，故用过去分词。Unless invited to speak = Unless you are invited to speak. 答案为A。