#include<bits/stdc++.h>
#define pll pair<ll, ll>
using namespace std;
typedef long long ll;
#define int ll

const int N = 1e5+10;
const int M = 1e6 + 7;

int n, m, vis[N], mx;
vector<ll> edge[N];
pll in[N];


void init(){
    mx = sqrt(n);
    if (mx * mx < n)mx++;
    for (int i = 1; i <= n; i++)edge[i].clear(), vis[i] = 0, in[i] = {0, i};
}

inline void solve(){
    cin >> n >> m;
    init();
    for (int i = 1, u, v; i <= m; i++){
        cin >> u >> v;
        edge[u].push_back(v);
        edge[v].push_back(u);
        in[u].first++, in[v].first++;
    }
    sort(in + 1, in + 1 + n, greater<pll>());
    queue<pll> q;
    for (int i = 1; i <= mx; i++){
        vis[i] = 1;
        q.push({0, in[i].second});
    }
    while (q.size()){
        pll temp = q.front();
        q.pop();
        ll x = temp.second, dis = temp.first;
        if (dis == mx)continue;
        for (auto i : edge[x]){
            if (vis[i])continue;
            vis[i] = 1;
            q.push({dis + 1, i});
        }
    }
    ll flag = 1;
    for (int i = 1; i <= n; i++){
        if (!vis[i])flag = 0;
    }
    if (flag){
        cout << mx << endl;
        for (int i = 1; i <= mx; i++)cout << in[i].second << " ";
        cout << endl;
    }
    else cout << -1 << endl;
}



signed main(){
    ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
    int WTN666=1;cin>>WTN666;
    while(WTN666--){
        solve();
    }
}