锅炉受热面金属遭破坏的原因一般是指（）、（）、（）、（）等。
参考答案：磨损、腐蚀、蠕变、应力
在一张纸上任意画两个大小相等的圆，这两个圆组成的图形至少有几条对称轴？[ ]
A．0
B．1
C．2
D．无数
In a regulated industry, the introduction of a marginal cost pricing role that maximizes total surplus would most likely be viewed as:（）
A. inefficient but leaves the natural monopoly incurring an economic loss.
B. inefficient and allows the natural monopoly to realize an economic gain.
C. efficient but leaves the natural monopoly incurring an economic loss.
