#include<bits/stdc++.h>

using namespace std;

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()

using ll = long long;

const int N = 2e5 + 5;

vector<int> gr[N];
int d[N], choose[N];
int need;
mt19937 gen;
void dfs(int v, int di) {
    int ch = 0;
    for (int u: gr[v]) {
        if (d[u] == d[v] + 1) {
            ch++;
        }
    }

    if (ch == 0 && di > need) {
        choose[v] = 1;
        return;
    }

    if (di > need * 2) {
        choose[v] = 1;
        di = 0;
    }

    for (int u: gr[v]) {
        if (d[u] == d[v] + 1) {
            dfs(u, di + 1);
        }
    }
}

void solve() {
    int n, m;
    cin >> n >> m;
    fill(d, d + n, N);
    for (int i = 0; i < n; i++) {
        gr[i].clear();
        choose[i] = 0;
    }
    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        u--, v--;
        gr[u].push_back(v);
        gr[v].push_back(u);
    }
    int st = gen() % n;
    queue<int> q;
    d[st] = st;
    q.push(st);
    vector<int> used(n, 0);
    while (!q.empty()) {
        int v = q.front();
        q.pop();
        for (int u: gr[v]) {
            if (used[u] || d[u] <= d[v] + 1)
                continue;
            d[u] = d[v] + 1;
            q.push(u);
        }
    }

    need = ceil(sqrt(n));
    choose[st] = 1;
    dfs(st, 0);
    vector<int> ans;
    for (int i = 0; i < n; i++) {
        if (choose[i])
            ans.push_back(i + 1);
    }
    if (sz(ans) > need) {
        cout << "-1\n";
    } else {
        cout << sz(ans) << '\n';
        for (int I: ans) {
            cout << I << ' ';
        }
        cout << '\n';
    }


}

int main() {
#ifndef LOCAL
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
#else
    freopen("input.txt", "r", stdin);
#endif
    gen.seed(66);
    int t;
    cin >> t;
    while (t--)
        solve();
    return 0;
}