Don't go too far into the sea, children, or the waves will _____ you off your feet.
Steel and iron can be used ______ bridges as well as machines.
Children are always ______, and usually think differently from adults.
I’m sure you ______ to us just now, because we already know the truth.
I have many friends, __________some are businessmen。A of them B of whose C of which D of whom
He had been ill for a long time and _________ he was left behind in his work。