------I think we should buy a car now so that it will be convenient for us.

------      , but do you think we have so much money?

A．You are dead right

B．You are joking

C．I don’t believe it

D．Come off it

长期平均成本曲线是无数短期平均成本曲线的包络线，所以( )。

A．长期平均成本与最低短期平均成本相等

B．长期平均成本与最低短期平均成本的平均数相等

C．长期平均成本只有在一个点上与最低短期成本平均成本相等

D．长期平均成本与短期平均成本相等