We can never expect ____ bluer sky unless we create _____less polluted world.
-----Did you remember to pay the telephone bill?------The telephone bill? ______.
There is a 20 minute parking _____ here, so we must get back to the car very quickly.
Radium salts, which have been found to _____ in bones, give rise to cancers of the bone.
It has been laid down that each student must _____ a composition to the teacher once a week.
I shall take you back to Beijing _____ you are well enough to travel.