请阅读 Passage 1，完成第21~25小题 。Passage 1As regards social convent

请阅读 Passage 1，完成第21~25小题 。Passage 1As regards social conventions，we must say a word about the well-known English class system. This is an embarrassing subject for English people，and one they tend to be ashamed of ，though during the present century class-consciousness has grown less and less，and the class system less rigid.But it still exists below the surface.Broadly speaking，it means there are two classes，the middle class and theworking class. (We shallignore for a moment the old upper class，including the hereditary aristocracy，since it is extremely small in numbers; but some of its members have the right to sit in the House of Lords，and some newspapers take surprising interest in their private life.) The middle class consists chiefly of well-to-do businessmen and prof essional people of all kinds.The working class consists chiefly of manual and unskilled workers.The most obvious difference between them is in their accent.Middle-class people use slightly varying kinds of received pronunciation which is the kind of English spoken by BBC announcers and taught to overseas pupils.Typical working-class people speak in many different local accents which are generally felt to be rather ugly and uneducated.One of the biggest barriers of social equality in England is the two-class education system.To have been to a so-called public school immediately marks you out as one of the middle class.The middle classes tend to live a more formal life than working-class people，and are usually more cultured.Their midday meal islunch and they have a rather formal evening meal calleddinner，whereas the working man's dinner，if his working hours permit，is at midday，and his smaller，late-evening meal is called supper.As we have said，however，the class system is much less rigid than it was，and for a long time it has been government policy to reduce class distinctions.Working-class students very commonly receive a university education and enter the professions，and working-class incomes have grown so much recently that the distinctions between the two classes are becoming less and less clear. However，regardless of one's social status，certain standards of politeness are expected of everybody，and a well-bred person is polite to everyone he meets，and treats a laborer with the same respect he gives an important businessman.Servility inspires both embarrassment and dislike.Even the word sir，except in school and in certain occupations (e.g.commerce，the army，etc.) sounds too servile to be commonly used.Why isn't the word sir commonly used in Britain?