单项选择题

In the United States, teaching is very important. If teachers do not teach well, students complain. If many students do no understand, people think that the teacher does not do a good job. The teacher has big responsibility to make sure the students understand. In a sense, students are consumers and teacher is offering services.
Students have the right to evaluate their teachers, and they usually do so at the end of each course. That evaluation includes lots of aspects of teaching, such as explanation, preparation, using good examples, answering questions, and Organization of classes.
Students am supposed to read required textbook and recommended books before classes. Without reading them, they will find it very difficult to understand class. Basic courses on doing research and writing papers are taught at the beginning of the program. They teach things very concretely and clearly. Students are expected to apply what they have learned to their studies.
Classes are usually divided into lectures and seminars. There are some lecture classes, where the teacher just gives a lecture, and students ask questions at the end. Most graduate level classes are lectures and discussions. Participating in discussions is very important. Higher level, classes involve very little lecturing. They emphasize discussion and presentation by the students.

Higher level classes are taught mainly through（）

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