单项选择题

A.q:=p↑.link; p↑.link:=q↑.link;
B.p↑.link:=q↑.link; q:=p↑.link;
C.q↑.link:=p↑.link;p↑link:=q;
D.p↑.link:=q; q↑.link:=p↑.link;