______ their homework, these three students have to stay in the classroom for another thirty minutes.

A．Not finishing

B．Not finished

C．Having not finished

D．Not having finished

下列哪项价格歧视最显著的特征是价格与顾客身份无关（）.

A.完全价格歧视

B.非线形定价

C.三级价格歧视

D.一级价格歧视