In English，people use many expressions with the word dog．People in Britain and America love their dogs and treat them well．They take their dogs for walks，let them play outside and give them good food and medical care．However，dogs without owners to care for them lead a different kind of life．The expression，to lead a dog's life，means living an unhappy life．

Some people say we live in a dog-eat-dog world．That means many people are competing for the same thing，like good jobs．They say that to be successful，a person has to work very hard； to put it in another way，he has to work like a dog．Such work can make people dog tired．And the situation will be even worse if they become sick as a dog．

Sometimes we would let a sleeping dog lie，which means we'd better not trouble a person who is likely to cause problems．And if someone says you are a lucky dog，he is just saying you're very lucky．

Very often，people say every dog has its day．This means that every person enjoys a successful period during his or her life．To achieve one's goal，people often have to learn new skills．Yet some people say that you can never teach an old dog new tricks．They believe that older people do not like to learn new things and will not change they way they do things．

Dog expressions are also used to describe the weather．The dog days of summer are the hottest days of the year．A rainstorm may cool the weather．But we do not want it to rain too hard．In other words，we do not want it to rain cats and dogs．

39．What does the passage mainly talk about？___

A．How to live with dogs

B．Expressions about dogs．

C．People's love for dogs．

D．Ways to train dogs．

40．According to what you have read above，working too hard can make people___．

A．let a sleeping dog lie．

B．become a lucky dog

C．live in a dog-eat-dog world

D．work like a dog

41．What can we learn from the passage？___

A．It m