在关系数据库中，允许 ______。

A．不同属性来自同一个域

B．同一个关系中两个元组相同

C．同一列的数据类型不同

D．属性可以进一步分解

—What did the teacher say just now?

—Sorry, I didn’t hear it. I          out of the window.[ ]

A. look

B. looked

C. am looking

D. was looking