#include <bits/stdc++.h>

using namespace std;

const int maxn = 2e5 + 10, oo = 1e9;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rand(int l, int r) {
    return uniform_int_distribution<int>(l, r)(rng);
}

int n, m;
int dis[maxn], in[maxn];
vector <int> graph[maxn];

void bfs(int root) {
    for (int i=0; i <n; i++) dis[i]=oo;
    queue <int> q;
    for (int i=0; i < n; i++) if (in[i]) dis[i]=0, q.push(i);
    while (!q.empty()) {
        int u = q.front(); q.pop();
        for(int&v : graph[u]) {
            if (dis[v] > dis[u]+1) {
                dis[v]=dis[u]+1;
                q.push(v);
            }
        }
    }
}
void clean() {
    for (int i=0; i < n; i++) {
        graph[i].clear();
		in[i]=0;
        dis[i]=oo;
    }
}
int main() {
    ios_base::sync_with_stdio(0), cin.tie(0);
    #ifdef LOCAL
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
    #endif // LOCAL

    int tt; cin >> tt;
    while (tt--) {
        cin >> n >> m;
        for (int i=0; i<m; i++) {
            int u, v; cin >> u >> v;
            u--, v--;
            graph[u].push_back(v);
            graph[v].push_back(u);
        }

        bfs(rand(0, n-1));
        int limit = ceil(sqrt(n));
        vector <int> taken;
        while (taken.size() < limit) {
            int w=-1;
            for (int i=0; i<n; i++) {
                if (in[i]) continue;
                if (w == -1 || dis[i]>dis[w]) w=i;
            }
			if (w == -1) break;
            taken.push_back(w);
            in[w]=1;
            bfs(w);
        }

        bool ok=1;
        queue <int> q;
        for (int i=0; i <n; i++) dis[i]=oo;
        for (int& u : taken) q.push(u), dis[u]=0;
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for(int& v : graph[u]) {
                if (dis[v] > dis[u]+1) {
                    dis[v] = dis[u]+1;
                    q.push(v);
                }
            }
        }
        for (int i=0; i<n; i++) ok &= dis[i]<=limit;
        if (!ok) cout << -1 << endl;
        else {
            cout << taken.size() << endl;
            for (int& i : taken) cout << i+1 << ' ';
            cout << endl;
        }
        clean();
    }
    return 0;
}