随着产量的增加，平均固定成本AFC（    ）。
A．先递减后递增     B．先递增后递减
C．一直趋于递减     D．一直趋于递增 
A、C
B、A
C、B
D、D

A．It can catalyze the same chemical reaction
B．generally, it is an oligoenzyme
C．It can exist in different tissues of the same individual
D．It has different physical and chemical properties