— What’s the matter with you?

— Oh, once again I had my best friend ___, because the expression on his face ____ his anger.

A．hurting; betraying

B．to be hurt; to betrayed

C．hurt; betrayed

D．being hurt; betraying

轴流风机的工作原理是基于流体力学的（）

A.升力原理

B.离心理论

C.平衡理论