---Did you notice Tom was busy all day in his office?---Oh, yes, he has just finished his business tour of Chicago. He has a lot of work to ___.
I'll come to see you if _____.
—Must I turn off the gas after cooking ? —Of course . You can never be _______ careful with that.
_____ is known to everybody, the moon travels round the earth once every month.
It is so difficult a problem that _____ student in this class _____ work it out.
Shirley _____ a book about China last year, but I don't know whether she has finished it.