当改变气体燃烧器所用的气体时，只需重新进行调整（）与（）的配比，一般不必更换任何零件。
参考答案：空气；燃气
钢的含义是指平均含碳量为( )%。
A．40
B．4.0
C．0.4
D．0.04
Which of the following is least relevant when explaining why monopoly firms can earn positive economic profits over the long term（）
A. The ability to use price discrimination.
B. The existence of economies of scale.
C. Control over production input resources.
