— Do you know _________?      —No, I don’t know.

A．where he’s from

B．where is he come

C．where does he come from

D．where comes he from

数据库中取出关系中的某些列，并取消重复的元组的关系运算为（）

A．列取消运算

B．投影运算

C．联结运算

D．选择运算