降管必须采取绝热措施，其目的是（）。
A、防止受热爆管
B、防止烟气冲刷
C、防止高温腐蚀
D、保证水循环可靠
参考答案：D
In my opinion, all Mr. White ___ good to his students in his class at present. He is very strict in their study.
A．does does does
B．does do do
C．does does do
D．did do does
在一直线轨道上的甲、乙两列火车，以甲车为参照物，得出乙车向东运动的结论．若以地面为参照物，乙车的运动情况是（ ）
A．一定向东运动
B．一定向西运动
C．一定静止不动
D．向东、向西运动或静止均有可能
