根据短文理解，选择正确答案。
In England nobody under eighteen years old is allowed to drink in a bar（酒吧）. Mr. Thompson often went to a bar near his house. But he never took his son, Tom, because he was too young. Then when Tom had his eighteenth birthday, Mr. Thompson took him to his usual bar for the first time. They drank for an hour. Tom drank a bit. Then Mr. Thompson said to his son, "Now, Tom, I want to teach you a useful lesson. How do you know when you've had enough? Well, I'll tell you. Did you see those two lights at the end of the bar? When they seem to become four, you've had enough and should go home."
"But, dad, "said Tom, "I can only see one light at the end of the bar."
