In the United States, teaching is playing a very important role in society. If teachers do not teach well, students could complain about it to the administrative offices in the university. If many students do not understand, people think that the teacher does not do a good job. The teacher has big responsibility to make sure students understand. Ina sense, students are consumers and the 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 should be able to make objective judgment about the teacher.Students are supposed to read required textbooks and recommended books before class. With- out reading them, they will find it very difficult to understand the class. Basic courses on doing re- search 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 in classes to their studies.Classes are usually divided into a series of lectures and seminars (研讨会)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（）

A. giving a lot of lectures to students
B. involving students in discussions
C. asking students questions in class
D. answering students' questions in class