- The water was left running all night long.

- Yes. It seemed that John ___. He was the last one to leave the lab.

A．was to be blamed

B．was to blame

C． had been blamed

D． should be blamed

数据库系统其内部分为三级模式，即概念模式、内模式和外模式。其中，______是用户的数据视图，也就是用户所见到的数据模式。