In the United States, teaching is very important. If teachers do not teach well, students complain. If many students do not understand, people think that the teacher does not do a good job. The teacher has big responsibilities to make sure students understand. In a 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 are supposed to read required textbooks and recommended books before class. Without reading them, they will find it very difficult to understand the class. Basic courses on doing research and writing papers are taught at the beginning of the program. They teach things very concretely (具体的) and clearly. Students are expected to apply what they have learned to their studies.Classes are usually divided into lectures and seminars (研究讨论会). 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. When a program starts, students should first（）

A. learn how to do research and write papers
B. have some learning experience
C. learn about the course concretely and clearly
D. know how to participate in discussion