【单选题】
有关系R和S,R∩S的运算等价于()。
A、S一(R—S)
B、R—(R—S)
C、(R—S)∪S
D、R∪(R—S)
A、S一(R—S)
B、R—(R—S)
C、(R—S)∪S
D、R∪(R—S)
A、s->next=r;r=s; B、r->next=s;r=s; C、s->next=f;f=s; D、f->next=s;f=s;