#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int N = 2e5 + 10;
int h[N], e[N << 1], ne[N << 1], idx;
bool vis[N];
bool d[N];
int ans[N];
int q[N], tt, hh;

void add(int a, int b) {
    e[++idx] = b, ne[idx] = h[a], h[a] = idx;
}

void bfs(int dis, int x) {
    tt = hh = 0;
    q[0] = x;
    int dep = -1;
    int last_tt = 0;
    while (hh <= tt) {
        int t = q[hh++];
        if (hh == N) hh = 0;
        vis[t] = true;
        
        for (int i = h[t]; i; i = ne[i]) {
            int j = e[i];
            if (tt == N) tt = 0;
            q[++tt] = j;
        }
        if (hh - 1 == last_tt) {
            dep++;
            last_tt = tt;
        }

        if (dep == dis) {
            break;
        }
    }
}

void solve() {
    
    int n, m;
    scanf("%d%d", &n, &m);
    for (int i = 0; i < m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        add(u, v), add(v, u);
    }
    int num = 0;
    int sqrtn = ceil(sqrt(n));
    for (int i = 1; i <= n; i++) {
        if (num == sqrtn) break;
        if (vis[i]) continue;
        ans[num++] = i;
        bfs(sqrtn, i);
    }
    bool flag = 0;
    for (int i = 1; i <= n; i++) {
        if (!vis[i]) {
            puts("-1");
            return;
        }
    }
    printf("%d\n", num);
    for (int i = 0; i < num; i++) {
        printf("%d ", ans[i]);
    }
    puts("");
}

int main() {
    int T;
    scanf("%d", &T);
    while (T--) {
        solve();
        if (T) memset(h, 0, sizeof h), memset(vis, false, sizeof vis);
        idx = 0;
    }
    return 0;
}