A、At the airport. B、By plane. C、Taking the air. D、On radio and TV. DWhat does "on the air" here mean?

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 vas 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 received advice at one of the 283 study centers in the country. In 36 weeks of the year he has to send written work to a "tutor", the person who guides his studies. He must also spend 3 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 4 students passed their examinations. If they do this every year, they will finish their studies in 4 or 5 years.