A.p ->prior =q;q->next=p;p ->prior ->next=q;q ->prior=p ->prior;
B.q ->prior=p ->prior;p ->prior ->next=q;q->next=p;p ->prior =q->next;
C.q->next=p;p ->next=q;q->prior ->next=q;q->next=p;
D.p ->prior ->next=q;q->next=p;q ->prior =p->prior;p ->prior =q;