13. Don’t play by the river _______ you fall in and drown!
Mary took _____ advantage of the opportunity and tried to make _______ good impression on her new boss.
--If we don’t do our best to reduce pollution, man’s history won’t last long.--_________.
He likes sports, ________ I’d rather play games on computers.
________ he is the last man I wanted to see, I did my best to help him.
He tried his best to solve the problem, ________ difficult it was.