引起急性胆囊炎最主要的病因是（）
A.胆囊内蛔虫
B.严重创伤
C.胆囊结石
D.胆囊扭转
E.胆囊管狭窄
参考答案：C
计算：（a2）4÷a3÷a=______；（a2b-ab2）•（-5a）=______．
Student loan is supposed to provide financial support for people who would ________ not be able to go to college.
A．otherwise
B．meanwhile
C．therefore
D．nevertheless
