资料题：American university students are usually under pressure of work because（  ）。

Normally a student must attend a certain number of courses in order to graduate, and eachcourse which he attends gives him a credit(学分) 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 (学期).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 ispossible to spread the period of work for the degree over a longer period.It is also possible for astudent 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 that he follows a student is given a grade, which is recorded, and the record is available for the student to show to prospective employers.All this imposes a constant pressure and strain of work, but in spite of this 8ome 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.Any student who is thought to have broken the rules, for example, by cheating has to appear before a student count.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.