[单选]  以下函数为链栈的进栈操作，x是要进栈的结点的数据域，top为栈顶指针struct node{  ElemType data;struct node *next;};structnode*top;voidPush(ElemTypex)           {         structnode*p;         p=(structnode*)malloc __(1)__;         p->data=x;                                      __(2)__;                                    __(3)__;                                           }