#include<bits/stdc++.h>
using namespace std;
#define pii pair<int,int>
#define pb push_back
#define fi first
#define se second
#define endl "\n"


vector<int> v[200005];
int par[200005];
int Find(int x)
{
    if(par[x]==x) return x;
    return par[x] = Find(par[x]);
}
void Union(int a,int b)
{
    par[Find(a)] = Find(b);
}

int sz[200005], sqr;
set<int> ans;

void dfs(int u, int p) {
    sz[u] = 1;
    for(int child: v[u]) {
        if(child == p) continue;

        dfs(child, u);
        sz[u] += sz[child];
    }
    if(sz[u] > sqr) {
        ans.insert(u);
        sz[u] = 0;
    }
}

void solve()
{
    int n,m;
    cin>>n>>m;
    vector<pii> edge;
    for(int i=0;i<m;i++)
    {
        int a,b;
        cin>>a>>b;
       edge.pb({a,b});
    }
    for(int i=0;i<=n;i++) par[i]=i;
    for(int i=0;i<m;i++)
    {
        int a=edge[i].first,b=edge[i].second;
        if(Find(a) != Find(b))
        {
            v[a].pb(b);
            v[b].pb(a);
            Union(a,b);
        }

    }
    /////////////////////////////////////////////////////////
    sqr = sqrt(n);
    while(sqr*sqr < n) sqr++;

    dfs(1, 0);
    ans.insert(1);

    if(ans.size() > sqr) {
        cout<< -1 << endl;
        return;
    }
    cout<<ans.size()<<endl;
    for(int i:ans) cout<<i<<' ';
    cout<<endl;
    for(int i=0;i<=n;i++) {
        sz[i] = 0;
        v[i].clear();
    }
    ans.clear();
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    int tc;
    cin>>tc;
    while(tc--)
    {
        solve();
    }
}