柴油机处在完好的技术状态时，其排气烟色应为（）。
A.黑色
B.蓝色
C.灰色
D.白色
参考答案：C
— I telephoned him twice and I couldn't get through to him.
— I think that the line might have been out of order, ______? [ ]
A. don't you
B. wasn't it
C. do you
D. hadn't it
稀硝酸装置停车前应该（）。
A.制订完善的停工方案
B.氨蒸发器氮气置换
C.做好物品准备
D.学习消防知识
