设有关系模式：课程(课程号，课程名，学分)和课程参考书(书号，书名，课程号)，要求课程参考书关系中的课程号属性值或者为空，或者等于课程关系中某个元组的主码列的值。能够实现此要求的完整性约束是（）约束。

A．select dname from dep right join student as s on dep. dno= s. dno where sno is null
B．select dname from dep right join student as s on dep. dno=s, dno where dep. dno is null
C．select dname from dep left join student as s on dep. dno=s, dno where sno is null
D．select dname from dep left join student as s on dep. dno=s, dno where dep. dno is null

A、分片模式
B、分配模式
C、概念模式
D、外模式