Unless to speak you should remain silent at the conferenceA. 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：本题考查过去分词在省略结构中的用法。Unless invited相当于 Unless you are invited。