Passengers must not leave their seats ________ the plane comes to a full stop.

As a teacher, he has never given his students so difficult a problem ____ they can’t work out

u like working as a waiter—Yes, but I get tired ______ standing thewhole day．

Do you know __________ they got to know each other? — It was last year ________ they both taught English in China.

Would you please speak more slowly I can hardly ______you.