单项选择题
A.总成本等于总变动成本B.总成本大于总变动成本C.总成本是否大于总变动成本不确定D.平均变动成本的最低点恰好在总成本曲线的拐点上
A.Have met for several time.B.Classmates.C.Don't know each other before.D.Friends.