单项选择题
A.p↑.rlink ↑.llink:=q;p ↑.rlink:=q;q ↑.llink:=p;q↑.rlink:=p ↑.rlink;
B.p↑.llink ↑.rlink:=q;p↑.llink:=q;q↑.rlink:=p;q↑.llink:=p ↑.llink;
C.q↑.llink:=P;q ↑.rlink:=p↑.rlink;p↑.rlinkl:=q;p↑.rlink:=q;
D.q↑.rlink:=p;q ↑.llink:=p ↑.llink;p ↑.llinkl:=q;p ↑.llink:=q;