初次启动非自动控制的空压机前无须做的是（）。
A．接通冷却水管路
B．手动卸载
C．开启排气管路上的阀
D．开启吸气截止阀
参考答案：D
— Look at the boy playing basketball on the ground. Is it George?
— It ________ be him. He told me he would play basketball after class, but he’s not sure.
A．mustn’t
B．must
C．can’t
D．may
什么是人为噪声？
