- Why didn’t Sally play the violin at the concert last night?

- She said that her hand hurt,but that was only a(n) ____.I saw her play tennis just now.

A．excuse

B．matter

C．result

D．expression

使金属产生疲劳的载荷为（），综合机械性能是指材料的强度，硬度，塑性，韧性达到良好配合的机械性能。

A、静载荷

B、交变载荷

C、轻载荷

D、重载荷