万向传动装置有何作用？
参考答案：
（1）传递相交成角的两轴的旋转力矩
（2）在相交两轴夹角改变时也能正常的传递扭矩
（3）降低了安装准确度，易于对中。
If you really have to leave during the meeting, you'd better leave _____ the back door. [ ]
A. for
B. by
C. across
D. out
简述文献零增长理论。
