联合单位法计算保本量的关键是（）。
A.各产品的销售单价
B.各产品之间的实物量比例
C.各产品的变动成本
D.各产品的保本量
参考答案：B
-----He should have come to join in the thorough cleaning.
-----He________, I think. It’s not his turn.
A．needn’t have to
B．needn’t have
C．didn’t have to
D．doesn’t need to
生态学效度
