A man comes home from work late. His five-year-old son is waiting for him at the door. “Daddy, may I ask you a question?” “Sure. What is it?” says the man. “Daddy, how much do you make an hour?” “ If you must know ,I make $20 an hour. “ “Oh,” the little boy says , “Daddy, may I please borrow $10?” “ Here you are, but why do you want to borrow money?” the father asks. “Because I didn't have enough money, but now I do.” The little boy says happily. “Daddy, I have $20 now, can I buy an hour of your time？Please come home early tomorrow. I would like to have dinner with you.” The man makes twenty dollars a day. (       )