焊接之前，应将坡口两侧20mm范围内的（）清理干净。
A、铁锈、油污、氧化皮等污物
B、泥、表面金属
C、其他金属成分
D、焊疤
参考答案：A
已知消防水枪射流量为3L/s，采用衬胶水龙带，水龙带长25m，试计算栓口处所需压力。水龙带比阻A2值.
People were afraid to leave their house, （） the police had been ordered to stand by in case of emergency, they were just as confused and helpless as anybody else.
A. and even if
B. but since
C. however after
D. for though
