—Look! He’s running so fast!

—Hard to______ his legs were once broken.

A．know

B．imagine

C．realize

D．find

高周疲劳是一种常见的低应力、高寿命的疲劳破坏，如（）的裂纹和断裂。

A．气缸盖

B．气缸套

C．曲轴

D．船体