下面Prim算法程序中，横线处应该填入的是（ ）。
#include <iostream>#include <vector>#include <algorithm>using namespace std;int prim(vector<vector<int>> & graph, int n) { vector<int> key(n, INT_MAX); vector<int> parent(n, -1); key[0] = 0; for (int i = 0; i < n; i++) { int u = min_element(key.begin(), key.end()) - key.begin(); if (key[u] == INT_MAX) break; for (int v = 0; v < n; v++) { if (__________) { // 在此处填入选项 key[v] = graph[u][v]; parent[v] = u; } } } int sum = 0; for (int i = 0; i < n; i++) { if (parent[i] != -1) { cout << "Edge: " << parent[i] << " - " << i << " Weight: " << key[i] << endl; sum += key[i]; } } return sum;}int main() { int n, m; cin >> n >> m; vector<vector<int>> graph(n, vector<int>(n, 0)); for (int i = 0; i < m; i++) { int u, v, w; cin >> u >> v >> w; graph[u][v] = w; graph[v][u] = w; } int result = prim(graph, n); cout << "Total weight of the minimum spanning tree: " << result << endl; return 0;}