Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Instructors at American colleges and universities use many different teaching methods. Some instructors give assignments every day. They grade homework. Students in their classes have to take many quizzes, a midterm exam, and a final test. Other instructors give only writing assignments. Some teachers always follow a course outline and usually use the textbooks. Others send students to the library for assignments.The atmosphere in some classrooms is very formal. Students call their instructors “Professor Smith”, “Mrs. Jones”, and so on. Some teachers wear business clothes and give lectures. Other classrooms have an informal atmosphere. Students and teachers discuss their ideas. Instructors dress informally, and students call them by their first names. American teachers are not alike in their teaching styles.At most American colleges and universities, facilities for learning and recreation are available to students. Students can often use recorders, video machines, and computers at libraries and learning centres. They can buy books, notebooks, and other things at campus stores. They can get advice on their problems from counsellors and individual help with their classes from tutors. Students can relax and have fun on campus, too. Some schools have swimming pools and tennis courts. Most have snack bars or cafeterias. 

What is the main idea of the first paragraph?