She will tell us why she feels so strongly that each of us has a role ________ in making the earth a better

place to live. [ ]

A. to have played  

B. to play

C. to be played  

D. to be playing

数据模型按不同应用层次分成三种类型，它们是概念数据模型、 【5】 和物理数据模型。