--I believe that more effective measures have to be taken before we can breathe cleaner air.
--______. Air pollution has become too serious a problem.

My sister-in-law is the ______ of the information, so it can’t be false.

The father, ready to retire at an early age, hoped that his eldest son would ______ the family business.

Was it through Mary, ______ was working at a high school, ______ you got to know Tom?

______ to smoking for a considerably long time, he has much difficulty quitting it.