#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define in(a, b) for (ll i = (a); i <= (b); i++)                // in using i
#define inj(a, b) for (ll j = (a); j <= (b); j++)               // in using j
#define ink(a, b) for (ll k = (a); k <= (b); k++)               // in using k
#define inl(a, b) for (ll l = (a); l <= (b); l++)               // in using l
#define inr(a, b) for(ll i = (a); i >= (b); i--)                // in reverse
#define inrj(a, b) for(ll j = (a); j >= (b); j--)                // in reverse
#define inrk(a, b) for(ll k = (a); k >= (b); k--)                // in reverse
#define inrl(a, b) for(ll l = (a); l >= (b); l--)                // in reverse
#define tt ll tcs; cin>>tcs; while(tcs--)                       // include test cases
#define ina(arr,n) ll arr[(n+1)]={0}; in(1,n) cin>>arr[i]       // input arr of n elements
#define inv(vec,n) vector<ll> vec(n+1); vec[0]=0; in(1,n) cin>>vec[i]; // input vector of n elements
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define pll pair <ll,ll>
#define vpll vector <pll>
#define sll set <ll>
#define vll vector<ll>
#define mll map <ll,ll>
#define all(x) x.begin(), x.end()
const ll mod=1e9+7;
#define vvll vector<vll>
#define pref(p,a,n) vll p(n+1); in(1,n) p[i]=p[i-1]+a[i];
#define vec2(a,n,m) vvll a(n+1,vll(m+1))
// #define vec2(a,n,m,val) vvll a(n+1,vll(m+1,val))

#define vec3(a,l,m,n) vector<vvll>a(l+1,vvll(m+1,vll(n+1)));
// #define vec3(a,l,m,n,val) vector<vvll>a(l+1,vvll(m+1,vll(n+1,val)));
# define FAST ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
// DSU;
vll dsu_parent;
vll dsu_size;
void initiate_dsu(ll n){
    dsu_parent.clear();
    dsu_size.clear();
    dsu_parent.resize(n+1);
    in(1,n) dsu_parent[i]=i;
    dsu_size.assign(n+1,1);
}
ll find_set(ll v) {
    if (v == dsu_parent[v])
        return v;
    return dsu_parent[v] = find_set(dsu_parent[v]);
}
void make_set(ll v) {
    dsu_parent[v] = v;
    dsu_size[v] = 1;
}
 
void union_sets(ll a, ll b) {
    a = find_set(a);
    b = find_set(b);
    if (a != b) {
        if (dsu_size[a] < dsu_size[b])
            swap(a, b);
        dsu_parent[b] = a;
        dsu_size[a] += dsu_size[b];
    }
}
int main(){
    FAST
    tt{
        ll n,m;
        cin>>n>>m;
        initiate_dsu(n);

        vvll adj(n+1);
        in(1,m){
            ll a,b;
            cin>>a>>b;
            if(find_set(a)!=find_set(b)){
                union_sets(a,b);
                adj[a].push_back(b);
                adj[b].push_back(a);
            }
        }
        //tree
        vll dis(n+1);
        vll sett;
        ll mxdis=sqrt(n);
        if((mxdis*mxdis)>=n){

        }
        else{
            mxdis++;
        }

        function<void(int,int)>dfs=[&](int v, int p){
            for(auto x:adj[v]){
                if(x!=p){
                    dfs(x,v);
                    dis[v]=max(dis[v],1+dis[x]);
                }
            }
            if(dis[v]==mxdis){
                dis[v]=0;
                sett.push_back(v);
            }
        };
        dfs(1,0);
        if(sett.size()==0){
            sett.push_back(1);
        }
        if((sett.back()!=1)&&(adj[1].size()!=1)){
            ll mx=0;
            ll mn=n+1;
            for(auto x:adj[1]){
                mx=max(mx,1+dis[x]);
                mn=min(mn,1+dis[x]);

            }

                if((mx+mn)<=mxdis){

                }
                else{
                    sett.push_back(1);
                }
            

        }
        if(sett.size()<=mxdis){
            cout<<sett.size()<<endl;
            for(auto x:sett){
                cout<<x<<' ';
            }
            cout<<endl;
        }
        else{
        cout<<-1<<endl;;
        }
    }
}