单项选择题

Normally a student must attend a certain number of courses in order to graduate, and each course he attends gives him a credit which he may count towards a degree. In many American universities the total work for a degree consists of thirty-six courses each lasting for one semester. A typical course consists of three classes per week for fifteen weeks; while attending a university a student will probably attend four or five courses during each semester. Normally a student would expect to take four years attending two semesters each year. It is possible to spread the period of work for the degree over a longer period. It is also possible for a student to move between one university and another during his degree course, though this is not in fact done as a regular practice.
For every course, a student is given a grade, which is recorded, and the record is available for the student to show to possible future employers. All this puts a constant pressure and strain of work on him, but in spite of this some students still find time for great activity in student affairs. Elections to positions in student organizations arouse much enthusiasm. The effective work of maintaining discipline is usually performed by students who advise the academic authorities. A student who is thought to have broken the rules, should appear before a student court. With the enormous numbers of students, the operation of the system does involve a certain amount of activity. A student who has held one of these positions of authority is much respected and it will be of benefit to him later in his career.
Students are enthusiastic in student organizati6ns probably because they ______.