单选：锅炉的受热面上结有水垢后，由于水垢的导热性能比金属性能差（），就会影响锅炉的传热效率。

题目内容：
锅炉的受热面上结有水垢后，由于水垢的导热性能比金属性能差（），就会影响锅炉的传热效率。
A．几倍
B．十几倍
C．几十倍
D．百倍
参考答案：

在达西公式hf=[λ（L/D）]（v2/2g）中，"D"表示的是（）。

在达西公式hf=[λ（L/D）]（v2/2g）中，"D"表示的是（）。A．管道长度B．水力半径C．沿程阻力系数D．管道直径

分类：安全生产法及相关法律知识  题型：单选
查看答案

All but the tiniest of roads have to have names so they can be recognized on a m

All but the tiniest of roads have to have names so they can be recognized on a map,and so people can

分类：安全生产法及相关法律知识  题型：单选
查看答案

It is widely accepted that English is the global language of modern times.About

It is widely accepted that English is the global language of modern times.About three decades ago,Fr

分类：安全生产法及相关法律知识  题型：单选
查看答案

Eleanor Roosevelt was the wife of America′s 32nd president,Franklin Roosevelt.Sh

Eleanor Roosevelt was the wife of America′s 32nd president,Franklin Roosevelt.She helped her husband

分类：安全生产法及相关法律知识  题型：单选
查看答案