#include<bits/stdc++.h>
#define ll long long
#define ff first
#define ss second
#define pb push_back
using namespace std;

typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

vector<vector<int>> g;
vector<int> colors;
vector<int> dist;


int calc(int n){
	int lo = 1, hi = 10000;
	int ans;
	while(lo <= hi){
		int mid = (lo+hi)>>1;
		if(mid * mid >= n) {
			ans = mid;
			hi = mid - 1;
		} else {
			lo = mid + 1;
		}
	}
	return ans;
}


int main() {
	ios_base::sync_with_stdio(0),cin.tie(0);
	int t; cin >> t;
	while(t--){
		g.clear();
		colors.clear();
		dist.clear();
		int n,m; cin >> n >> m;
		g.resize(n), colors.resize(n), dist.resize(n);
		for(int i = 0; i < m;++i){
			int a,b; cin >> a >> b;
			--a,--b;
			g[a].pb(b);
			g[b].pb(a);
		}

		int upTo = calc(n);
		vector<int> taken;
		for(int i = 0, j = 1; i < n && j <= upTo;++i){
			if(!colors[i]){
				taken.pb(i);
				dist[i] = 0;
				queue<int> q;
				q.push(i);
				++j;
				colors[i] = j;
				while(q.size()){
					int dad = q.front(); q.pop();
					if(dist[dad] == upTo) continue;
					for(auto adj: g[dad]){
						if(colors[adj] != j){
							q.push(adj);
							colors[adj] = j;
							dist[adj] = dist[dad] + 1;
						}
					}
				}
			}
		}

		bool posi = 1;
		for(int i = 0; i < n && posi; ++i){
			posi = colors[i] != 0;
		}

		if(posi){
			cout << taken.size() << '\n';
			for(auto va:taken)
				cout << va + 1 << ' ';
			cout << '\n';
		} else 
			cout << -1 << '\n';

	}

	return 0;
}