---May we leave the classroom now?

---No, you      . You       to leave until the bell rings.

A．mustn’t, are allowed

B．don’t have to, are supposed

C．needn’t, aren’t allowed

D．can’t, aren’t supposed

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