#include <bits/stdc++.h> 
using namespace std;
#define f first
#define s second
int main(){
	long long zz;
	cin>>zz;
	while(zz--){
		long long n,m;
		cin>>n>>m;
		vector<long long> adj[n];
		vector<bool> visited(n,0);
		vector<long long> depth(n,0);
		set<long long> stt;
		long double d = sqrtl(n);
    	long long bor = d;
   	    if(bor < d) bor++;
		for(long long i=0;i<m;i++){
		long long x,y;
		cin>>x>>y;
		x--;y--;
		adj[x].push_back(y);
		adj[y].push_back(x);
		}
		stt.clear();
		set<int> poss;
		queue<long long> q;
		int y=0;
		q.push(y);
		depth[y]=0;
		stt.insert(y);
		while(!q.empty()){
			long long store=-1;
				while(!q.empty()){
					long long v=q.front();
					q.pop();
					visited[v]=1;
					if(depth[v]==bor){
						store=v;
						break;
					}
					for(auto x:adj[v]){
						if(!visited[x]){
							if(depth[v]==bor-1){
								poss.insert(x);
							}
						depth[x]=1+depth[v];
						q.push(x);
						}
					}
				}
				while(!q.empty()){
					q.pop();
				}
				if(store==-1){
					for(auto x:poss){
						if(!visited[x]){
							q.push(x);
							depth[x]=0;
							stt.insert(x);
							break;
						}
					}
				}
				else{
					q.push(store);
					stt.insert(store);
					depth[store]=0;
				}
		}
		if(stt.size()<=bor){
		cout<<stt.size()<<'\n';
		for(auto x: stt){
			cout<<1+x<<' ';
		}
		cout<<'\n';
		}
		else
		cout<<-1<<'\n';
	}
	
	return 0;
}