I’ll never forget the good trick my classmates ________ on me when I was in the primary school.

A．paid

B．acted

C．played

D．made

要求关系中的元组在组成主键的属性上不能为空值，这是

A．参照完整性规则

B．实体完整性规则

C．用户定义完整性规则

D．域完整性规则