#include<bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
#define PI acos(-1)
#define io ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define mem(a,b) memset((a),(b),sizeof(a))
typedef long long ll;
typedef unsigned long long ull;
#define int long long
#define db long double

const double eps = 1e-6;
const int mod = 1e9 + 7;
const int maxn = 2e5 + 10;
int n, m, d[maxn], vis[maxn];
vector<int> dep[maxn];
vector<int> edge[maxn];

void bfs(int u) {
    queue<int> que;
    que.push(u);
    vis[u] = 1;
    dep[1].push_back(u);
    while(!que.empty()) {
        int now = que.front();
        que.pop();
        for(int i = 0; i < edge[now].size(); i++) {
            int v = edge[now][i];
            if(vis[v] == 0) {
                vis[v] = vis[now] + 1;
                dep[vis[v]].push_back(v);
                que.push(v);
            }
        }
    }
}

void solve() {
    cin >> n >> m;
    for(int i = 0; i <= n; i++) d[i] = 0, vis[i] = 0, edge[i].clear(), dep[i].clear();
    int pos = 0, maxx = 0;
    for(int i = 0, u, v; i < m; i++) {
        cin >> u >> v;
        edge[u].push_back(v);
        edge[v].push_back(u);
        d[u]++; d[v]++;
        if(d[u] > maxx) {
            maxx = d[u];
            pos = u;
        }
        if(d[v] > maxx) {
            maxx = d[v];
            pos = v;
        }
    }
    bfs(pos);
    // cerr << pos << '\n';
    // for(int i = 1; i <= n; i++) {
    //     cerr << i << ": " << vis[i] << '\n';
    // }
    int len = (int)ceil(sqrt(n));
    vector<int> ans;
    for(int i = 1; dep[i].size() > 0; i += (len + 1)) {
        for(int j = 0; j < dep[i].size(); j++) {
            ans.push_back(dep[i][j]);
        }
    }
    if(ans.size() > len) {
        cout << -1 << '\n';
    }
    else {
        cout << ans.size() << '\n';
        for(int i = 0; i < ans.size(); i++) {
            cout << ans[i] << ' ';
        }
        cout << '\n';
    }
}

signed main(){
   io;
    int T = 1;
    cin >> T;
    while(T--) {
        solve();
    }
    return 0;
}