#include <bits/stdc++.h>
using namespace std;
 
mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
#define int long long
#define pb push_back
#define rep(i,a,b) for(int i = a; i < b; i++)
#define all(x) x.begin(),x.end()
#define in(a) for(int i = 0; i<a.size(); i++) cin>>a[i];
#define out(a) for(int i = 0; i<a.size(); i++) cout<<a[i]<<" ";
typedef vector<int> vi;
#define sqrt(x) sqrtl(x)
#define ret(a) cout<<a<<"\n"; return

int n, m; 
vector<vi> adj;
vi vis;
int limit;
set<int> s;

void dfs(int v, int dist){
    if(vis[v]) return;
    vis[v] = 1;
    if(dist == limit){
        s.insert(v);
        dist = 0;
    }
    for(auto &x: adj[v]){
        dfs(x, dist + 1);
    }

}

void solve(){
    cin>>n>>m;
    s.clear();
    adj = vector<vi> (n+1);
    vis = vi (n+1, 0);
    long double d = sqrtl(n);
    limit = d;
    if(limit < d) limit++;
    rep(i,0,m){
        int x, y;
        cin>>x>>y;
        adj[x].pb(y);
        adj[y].pb(x);
    }

    rep(i,0,100){
        vis = vi (n+1, 0);
        s.clear();
        int node = rand() % n;
        node++;
        s.insert(node);
        dfs(node, 0);
        if(s.size() <= limit){
            cout<<s.size()<<"\n";
            for(auto &x: s){
                cout<<x<<" ";
            }
            cout<<"\n";
            return;
        }
    }

    cout<<-1<<"\n";
    
}   
 
int32_t main() {    
    auto begin = std::chrono::high_resolution_clock::now();
    ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0); 
    int t = 1; 
    cin>>t;
    for(int i = 1; i<=t; i++){
        // cout<<"Case #"<<i<<": ";
        solve();
    }
    auto end = std::chrono::high_resolution_clock::now();
    auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
    cerr << "Time measured: " << elapsed.count() * 1e-6 << "ms\n"; 
    return 0;
}