It’s difficult to _______ who will be elected to chairman of the Student’s Union, for the two candidates are well-matched.
______ to smoking for a considerably long time, he has much difficulty quitting it.
Was it through Mary, ______ was working at a high school, ______ you got to know Tom?
The father, ready to retire at an early age, hoped that his eldest son would ______ the family business.
My sister-in-law is the ______ of the information, so it cant be false.
--I believe that more effective measures have to be taken before we can breathe cleaner air. --______. Air pollution has become too serious a problem.