As a teacher, I seldom give my students so difficult a problem ________ they can not work out.
The letter I have been looking forward to _________.
_______is known to us all is that the 2016 Olympic Games will take place in Rio de Janeiro.
--May I use your mobile phone? ---_________.
Many more policeman should be sent to ________them ________some endangered animals.
______ we have got the chance, we’d better make full use of it.