用户引入线与电力线（220V用户线）交叉间距不得小于（）厘米，跨越障碍物不得有托磨现象，否则应加以保护。
参考答案：40
"Not having returned home yet, _____", his father said to the police. [ ]
A. his mother got worried
B. his mother must be worried
C. I think he is missing
D. he must have got into some trouble.
成卷的未曝光的黑白摄影感光纸 ( )
A．21
B．22
C．23
D．25
