A、Taking the air. B、By plane. C、On radio and TV. D、At the airport. C细节题。原文中对“on the air”给予了解释：通过收音机，电视等媒介方式进行远程教学。

Some students at the Open University left school 20 years ago. Others are younger but all must be at least 21 years old. This is one example of how the Open University is different from all other universities. Its students must either work full-time or be at home all day, for instance, mothers of families. They do not have to pass any examinations before they are accepted as students. This is why the university is called "open". The university was started in order to help a known group— people who missed having a university education when they were young.
    The first name for the Open University was "The University of the Air". The idea was to teach "on the air", in other words, on radio and television. Most of the teaching is done like this. Radio and television have brought the classroom into people’s homes. But this, on its own, is not enough for a university education. The Open University student also receives advice at one of 283 study centers in the country. 36 weeks of the year he has to send written work to a "tutor", the person who guides his studies. He must also spend three weeks every summer as a full-time student. Tutors and students meet and study together, as in other universities. At the end of the Open University’s first year, the results were good. Three out of every four students passed their examinations. If they do this every year, they will finish their studies in four or five years.
Questions 19 to 22 are based on the passage you have just heard.
19. Why is the university called "open"?
20. What does" on the air" mean?
21. How many weeks must a person spend every summer as a full-time student?
22. How long will the students take to finish their studies?