#include<bits/stdc++.h>
using namespace std;
const int N = 2e5+10;
vector<int> v[N], e[N];
int dis[N], dep[N];
bool vis[N];
vector<int> ans;
int n, m, k;
int x, y;
int in[N];
int ed, mxdis, st;
void bfs(int st){
    mxdis = 0;
     queue<int> q;
     q.push(st);
     dis[st] = 0;
     vis[st] = 1;
     while(!q.empty()){
        int x = q.front();
        q.pop();
        for(auto y : v[x]){
            if(vis[y]) continue;
            vis[y] = 1;
            e[x].push_back(y);
            e[y].push_back(x);
            dis[y] = dis[x] +1;
             mxdis = max(mxdis,  dis[y]);
           // if(dis[y] < k){
                q.push(y);
            //}
       //     q.push(y);
        }
     }
}
int dfs(int x, int fa){
    if(e[x].size() == 0){
        dep[x] = 0;
        return 0 ;
    }
    int mx = 0;
    for(auto y : e[x]){
            if(y == fa) continue;
       mx = max(dfs(y, x), mx);
    }
    dep[x] = mx + 1;
    mx++;
    if(mx%k == 0){
       ans.push_back(x);
    }
    return mx;
}
bool cmp(int x,int y){
     return in[x] > in[y];
}
int main(){
   int tt;
   cin >> tt;
   while(tt--){
    for(int i = 1;i <= n;i++){
        v[i].clear();
        e[i].clear();
        vis[i] = 0;
        in[i] = 0;
        dis[i]=0;
    }    
    cin >> n >> m;
    ans.clear();

    k = ceil(sqrt(n));
    while(k * k < n) k++;
    for(int i = 1;i <= m;i++){
         cin >>x >> y;
         in[x]++;
         in[y]++;
         v[x].emplace_back(y);
         v[y].emplace_back(x);
    }
    if(n == 1){
        cout << "1\n1\n";
        continue;
    }
        mxdis = 0;
        for(int i = 1;i <= n;i++){
           if(in[i] > mxdis){
             st = i;
             mxdis = in[i];
           }
        }
        bfs(st);//cout << "-----------\n";
        for(int i = 1;i <= n;i++){
            if(dis[i] == mxdis/2){
                st = i;
                break;
            }
        }
        dfs(st, 0);
        if(ans.size() > k){
            cout << "-1\n";
        }
        else{
           // if(ans.size() == 0) ans.push_back(st);
        cout <<ans.size() << "\n";
        for(auto x : ans){
            cout <<x << " ";
        }
        cout << "\n";
        }
   }
   return 0;
}