铝热焊预热可以消除模具中残余（），提高钢轨及模具的温度。
A.气体
B.熔渣
C.湿气
D.应力
参考答案：C
It is often________that one trouble never comes alone.In my rush to leave I forgot to take the
plan I had made the night before.[ ]
A. to say
B. saying
C. said
D. being said
与X线量无关的因素是（）。
A.管电流
B.管电压
C.给予X线管的电能
D.靶物质的原子序数
E.X线管阳极、阴极间的距离
