按社会平均成本定价的意义主要有( )。

A．是价值规律的要求

B．有利于促进社会主义市场经济的发展

C．有利于降低成本

D．有利于调动工人的积极性

E．有利于节约社会资源

— Has your former classmate come back from America?

— Yes , he________there for eight years.[      ]

A. has stayed  

B. stays 

C. stayed  

D. had stayed