----Could you lend me some of your paper? ----Sorry . Mine is _____.
Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.
Not until the early years of the 19th century ________ what heat was.
Tom kept quiet about the accident ________ lose his job.
A driver should __ the road when .
_______ in a friendly way, their quarrel came to an end.