#include<iostream>using namespace std;int sum(int x, int y) { return x + y; }int product(int x, int y) { return x*y; }void main(){?______________________;?int a, b, result;?cout << "a="; cin >> a;?cout << "b="; cin >> b;?pf= sum; result = pf(a, b);?cout << a << "+" << b << "=" << result << endl;?pf = product; result = pf(a, b);?cout << a << "*" << b << "=" << result << endl;}橫線處填什么 使之成立