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

．_________we are allowed- , we can’t drink any beer.

         A．As long as; to do                  B．Unless; to

         C．Until; doing                  D．On condition that; doing so

Unless________ to speak, you should remain silent at the meeting.

A beam of light will not bend round corners unless______to do so with the help of a reflecting device .  A.having been made       B.being made       C.made      D. you make 

In the lecture, little Tom kept silent all the time unless _________ to express his opinion.

A. he invited               B. he is invited           C. invited                     D. being invited