#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pll pair<int,int>
#define fi first
#define se second
typedef long long LL;
typedef unsigned long long ull;
int n,m,k;
const int N=1e6+10,M=5e4,mod=998244353;
int num[N],book[N];
int  res,cnt;

int h[N],e[N<<1],ne[N<<1],idx;

void add(int a,int b)
{
	e[idx]=b,ne[idx]=h[a],h[a]=idx;
	idx++;
}
vector<int> vt;
int fa[N];
int find_fa(int x)
{
	if(x!=fa[x]) return fa[x]=find_fa(fa[x]);
	return fa[x];
}

bool bk=0;

bool uni(int a,int b)
{
	a=find_fa(a),b=find_fa(b);
	if(a==b) return 0;
	fa[a]=b;
	return 1;
}

int dfs(int id,int f)
{
	int dis=0;
	for(int i=h[id];i!=-1;i=ne[i])
	{
		int j=e[i];
		if(j==f) continue;
		int tmp=dfs(j,id)+1;
		dis=max(tmp,dis);
	}
	if(dis>=k)
	{
		if(id==1) bk=1;
		vt.push_back(id);
		dis=0;
	}
	return dis;
}

int DFS(int id)
{
	DFS(2-id);
	return DFS(2-id)+1;
}

void solve() {
	bk=0;
	cin>>n>>m;
	k=ceil(sqrt(1.0*n));
	vt.clear();
	idx=0;
	for(int i=1;i<=n;i++) h[i]=-1,fa[i]=i;
	while(m--)
	{
		int a,b;
		cin>>a>>b;
		if(uni(a,b))
		{
			add(a,b),add(b,a);
		}
	}
	dfs(1,-1);
	if(!bk)
	vt.push_back(1);
	
	if(vt.size()>k){
		cout<<"-1\n";
		return;
	}
	
	cout<<vt.size()<<endl;
	for(auto it:vt) cout<<it<<" ";
	cout<<endl;
}


signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int T=1;
    cin>>T;
    while(T--)
    {
        solve();
    }
    return 0;
}
/*
 2
 4 3
 1 2
 2 3
 3 4
 6 7
 1 2
 2 3
 3 1
 1 4
 4 5
 5 6
 6 4
 
 1
 6 6
 1 2
 2 3
 3 4
 4 5
 5 6
 6 1
 
1
4 3
1 2
1 3
3 4
*/