—There is only 15 minutes left for work. What shall we do?—My God！There are always not any taxis when you want_______ 

The chairman thought________necessary to invite Professor Smith to speak at the meeting .

I always think that while we are in the world,we should try         .

Mr. Smith doesn’t like all that empty talk between Susan and Joan, which he thinks will definitely get them ________.

----Have you seen Tom and Mary?----I haven’t seen____ of them.