单位产品的总成本一般由两部分构成：（）。
A.单位成本
B.固定成本
C.平均成本
D.可变成本
参考答案：B, D
It is generally believed that teaching is _____ it is a science.[ ]
A. an art much as
B. much an art as
C. as an art much as
D. as much an art as
额定电压为6KV的聚氯乙烯绝缘电缆，其运行最高允许温度为（）℃。
A.50
B.60
C.70
D.80
