标准成本差异一般分为（）。
A.数量差异
B.价格差异
C.品种差异
D.结构差异
E.工时差异
参考答案：A, B
— I rang you at about nine, but there was no reply.
— Oh, that was probably________I was seeing the doctor. [ ]
A. why
B. when
C. what
D. that
煤气经过初冷器后发生哪些变化？
