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

问题补充：

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

A. invited

B. inviting

C. being invited

D. having invited

答案：

(答案→)A. invited

B. inviting

C. being invited

D. having invited

解析：考查非谓语动词作状语。You和invite构成被动关系，所以使用过去分词。句意：如果你没有被邀请发言，在会议上你应该保持沉默。