在一个单链表HL中，若要向表头插入一个由指针p指向的结点，则执行____。A:HL = p;  p->next = HL;  B:p->next = HL;  HL = p; C:p->next = HL;   p = HL; D:p->next = HL->next;  HL->next = p; 答案: p->next = HL;  HL = p;