#include<bits/stdc++.h>
using namespace std;

const int MN = 2e5 + 5;

vector<int> G[MN];

bool vis[MN];

void solve() {
    int n, m;
    cin >> n >> m;
    for(int i = 1; i <= n; ++i) G[i].clear();

    for(int i = 1; i <= m; ++i) {
        int x, y;
        cin >> x >> y;
        G[x].push_back(y);
        G[y].push_back(x);
    }

    int UP = (int)sqrt(n);
    while(1ll * UP * UP < n) ++UP;
    
    for(int i = 1; i <= n; ++i) vis[i] = 0;

    vector<int> ans;
    for(int i = 1; i <= n; ++i) if(!vis[i]) {
        ans.push_back(i);
        queue<pair<int, int>> q;
        q.push({i, 0});
        while(!q.empty()) {
            auto [x, d] = q.front();
            q.pop();
            if(d < UP) {
                for(auto &y : G[x]) {
                    if(!vis[y]) {
                        vis[y] = 1;
                        q.push({y ,d + 1});
                    }
                }
            }
        }
    }

    // assert(ans.size() <= UP);
    if(ans.size() > UP) {
        cout << "-1\n";
        return;
    }
    cout << ans.size() << "\n";
    for(auto &x : ans) cout << x << " " ; 
    cout << "\n";
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);

    int T;
    cin >> T;
    while(T--) {
        solve();
    }
}