柜面监测系统二级监测信息的反馈时限为发布后（）小时内。
A.3
B.18
C.24
D.48
参考答案：B
为了保证安全，防止阀盘运动中碰撞而产生火花，机械呼吸阀阀盘和阀座常用（）制成。
A、铸铁
B、橡胶
C、塑料
D、铸钢
--Have you _ your debt?
---Yes, we worked day and night to _____.
A．paid off, pay for them
B．paid for, pay them
C．paid, pay for them
D．paid for, pay them off
