A、A kind of exchange. B、A kind of business, C、A commercialized exchange. D、An international friendship association. C细节题。文章中提到互惠

In Britain the biggest number of working foreigners comes from the odd system known as "au pair".(21)Every week hundreds of girls arrive from strange capitals to live with English families. They are called au pairs.(21)An au pair girl stays with the family and has free food and lodging. In return she does light work in the house.(20)This system began as a genuine exchange, but now has become a commercial bargain between families who need help and girls who need to learn English. As learning languages has become more important, so the numbers involved have gone up.
    The system succeeds fairly well so far as teaching the girls English is concerned. But it is debatable how far it increases international friendship. There is a basic conflict of objectives — the hosts want cheap labor,(21)the girls want leisure and language. The girls often find it hard to meet English people of their own school ages, for the language schools and clubs are made up of other foreigners.
    In spite of its shortcomings, the au pair system has probably played some role in breaking down frontiers. The British Vigilance Society estimates that 15 percent of the girls are very unhappy, 15 percent very happy, and(22)70 percent more or less content; but only a small minority keep in touch after they go back home.
Questions 20 to 22 are based on the passage you have just heard.
20. What does the au pair system mean now?
21. What is not mentioned as an advantage of the au pair system for the girls?
22. How do the girls feel about the system?