简述轴封风机的工作原理？
参考答案：
利用叶轮旋转时产生的离心力使风机入口产生负压，将气体排出至大气。
Many Chinese students think science subjects are _______ foreign languages.
A．more difficult as
B．less difficult than
C．much difficult than
D．so difficult as
名词辨析题：超筋梁和少筋梁。
