#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include "../debug.h"
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif



const int N = 2e5+10;

vector<int> g[N];
int vis[N];
int dis[N];
int cnt[N];
int test;

int n, m, lim;

void cl(){
	test = 0;
	for(int i=1; i<=n; i++){
		g[i].clear();
		vis[i] = 0;
		cnt[i] = 0;
	}
}
void bfs(int root){
	test++;
	queue<int> q;
	q.push(root);
	dis[root] = 0;
	vis[root] = test;
	while(q.size()){
		int u = q.front();
		cnt[u]++;
		q.pop();
		for(auto& v : g[u]){
			if(vis[v] == test)continue;
			vis[v] = test;
			dis[v] = dis[u] + 1;
			if(dis[v] == lim)continue;
			q.push(v);
		}
	}
}
int main(){
	ios_base::sync_with_stdio(0); cin.tie(0);
	int T;
	cin>>T;
	while(T--){
		cin>>n>>m;
		lim = sqrt(n);
		while(true){
			if(lim * lim >= n )break;
			lim++;
		}
		debug(lim);
		for(int i=0; i<m; i++){
			int u, v;
			cin>>u>>v;
			g[u].push_back(v);
			g[v].push_back(u);
		}
		vector<int> ans;
		for(int i=1; i<=n; i++){
			if(vis[i])continue;
			bfs(i);
		}
		if(ans.size() > lim){
			cout<<-1<<'\n';
			cl();
			continue;
		}
		vector<int> v(n);
		iota(v.begin(),v.end(),1);
		sort(v.begin(),v.end(),[&](int i, int j){
				return cnt[i] < cnt[j];
				});
		reverse(v.begin(),v.end());
		test = 0;
		for(int i=1; i<=n; i++){
			vis[i] = 0;
		}
		for(int i=1; i<=n; i++){
			int node = v[i-1];
			if(vis[node])continue;
			ans.push_back(node);
			bfs(node);
		}
		if(ans.size() > lim){
			cout<<-1<<'\n';
			cl();
			continue;
		}
		cout<<ans.size()<<'\n';
		for(auto& i : ans) cout<<i<<' ';
		cout<<'\n';
		cl();
	}
	return 0;
}