放喷管线（）焊接。
A、不允许
B、不允许在现场
C、允许在现场
参考答案：B
In common with many people，I like white colour as it is a ________of purity.
A．symbol
B．sign
C．signal
D．mark
情绪行为障碍的成因，首推（）因素。
