正确戴安全帽必须注意两点：一是帽衬与帽壳应有一定间隙不能紧贴；二是（）。
A.应把安全帽戴正
B.必须系紧下颚带
C.不能坐在安全帽上
参考答案：B
When asked about their opinions about the schoolmaster, many teachers would prefer to see him step
aside _______ younger men. [ ]
A. in terms of
B. in need of
C. in favor of
D. in praise of
光纤中由于物质密度不均匀而造成的光散射，属于（）。
A.结构散射
B.瑞利散射
C.费涅耳散射
D.布里渊散射
