—I had been working till 4∶00 a.m. to prepare a report for my boss.—________you look very tired.
If _____ without following the doctor’s instructions, the medicine may have serious side effects.
________ you happen to see Mary, would you please ask her to come here?
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.