听力原文： Graduate students often work as teaching assistants while they study in the United States. Teaching assistants, or T.A., may get money or get to take classes for free, or both. A T.A. usually works about twenty hours each week. In some cases, the professors they assist have big undergraduate classes with hundreds of students. The professor gives one or two lectures a week, and teaching assistants lead smaller discussions at other times. They also give tests, grade work, provide laboratory assistance and meet with students who need help. Nowadays, many states have proposed to require that teaching assistants be able to speak English well enough for students to understand them. Universities have increased their efforts to deal with this problem. Our example school this week is the University of Southern California, in Los Angeles. The Institute of International Education says more foreign students go to USC than any other American university. The American Language Institute at USC provides training to help international teaching assistants improve their English. The university requires most non-native English speakers to pass a test before they can become a T.A. Those who went to college in an English-speaking country do not have to take the test. The same is true for those who scored at least twenty-seven on the speaking part of the TOEFL Internet-based test. What is the passage mainly about?