钢中声速约为5900m/s，2.5MHz声波的波长约为（）。
A.2.36mm
B.2.36cm
C.0.42mm
D.0.42cm
参考答案：A
He was __ twenty when he was gone.
A．hardly
B．scarecely
C．badly
D．almost
录囚制度始于( )
A．汉代
B．明朝
C．清代
D．宋代
