#include<bits/stdc++.h>
using namespace std;
const int N = 2e5+10;
vector<int> v[N], e[N];
int a[N], b[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;int _tmp;
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);
            dis[y] = dis[x] + 1;
            q.push(y);
        }
    }
}
int dfs(int x){
    if(e[x].size() == 0){
        dep[x] = 0;
        return 0 ;
    }
    int mx = 0;
    for(auto y : e[x]) mx = max(dfs(y), mx);
    ++mx;
    dep[x]=mx;
    if((mx == k || mx %(2*k)==0)){
       ans.push_back(x);
    }
    return mx;
}
int dfs_1(int x){
    if(e[x].size() == 0){
        dep[x] = 0;
        return 0 ;
    }
    int mx=0;
    for(auto y : e[x]) mx = max(dfs_1(y), mx);
    ++mx;
    return mx;
}

bool cmp(int x,int y){
     return in[x] > in[y];
}
int main(){
    int tt;
    cin >> tt;
    while(tt--){
        cin >> n >> m;
        ans.clear();
        k = ceil(sqrt(n));

        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";
        _tmp=dfs_1(st);
        dfs(st);

        // for(int i = 1;i <= n;i++){
        //     if(dis[i]%(2*k) == 0 && dep[i] > (4 * k - 1))
        //         ans.push_back(i);
        // }
        ans.push_back(st);
        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";
        }
        for(int i = 1;i <= n;i++){
            v[i].clear();
            e[i].clear();
            vis[i] = 0;
            in[i] = 0;
        }
   }
   return 0;
}
/*
16 15
1 2
2 3
3 4
4 5
5 6
6 7
7 8
8 9
9 10
10 11
11 12
12 13
13 14
14 15
15 16
*/