矿井铁路坡度大于（）‰时，禁止人力推矿车。
A.10
B.20
C.30
参考答案：A
发声体做无规则振动时发出的声音叫______．从环境保护的角度看，凡是妨碍人们正常休息、______和______的声音，以及对人们要听的声音产生干扰的声音都属于______．
Mr. Black must be worried about something, you can ______ it from the look on his face.
A．react
B．read
C．recognize
D．realize
