[单选题]下面属于起重机的不安全方向有（ ）。
A.下降方向
B.缩臂方向
C.幅度增加方向
D.幅度减小方向

Passage 2 Normally a student must attend a certain number of courses in order to graduate, and each course which he attends gives a credit which may matter toward 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 followed 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 affair
A. their academic performance will affect their future careers
B. they are heavily involved in student affairs
C. they have to observe university discipline
D. they want to run for positions of authority