题目内容：
Have you ever seen the advertisement: Learn a foreign language in six weeks, 21give your money back? Of course, it 22. happens quite like that. The only language 23 to learn is the mother language. And think 24 practice is needed for that. Beforethe Second World War people usually learned a foreign language 25 the literature (文学)of the country. Now most people want to 26 a foreign language. Every year millions ofpeople start learning 27 How do they do it? Some people try at home 28 books and tapes, others go to eveningclasses or watch TV programs. 29 they use the language only 2 or 3 times a week, learning it will 30 a long time, like language learning at school. A few people try to learn alanguage fast by studying for 6 or 31 hours a day. It's much easier to learn the language inthe country where it 32 . But most people are 33 to do this, and many people don'thave to do so. Machines and good books will be very 34 , but they can not do the students'work, 35 the language is learned quickly or slowly, it is hard work.
根据以上内容,回答题.
第21题应填入( ) A．so
B．or
C．and
D．but
参考答案：
答案解析：