#include <bits/stdc++.h>

using namespace std;
using pii = pair<int, int>;

const int maxn = 2e5 + 10;

int n, m, limit;
int vis[maxn], height[maxn], dis[maxn];
vector <int> graph[maxn] ,tree[maxn], ans;
void dfs(int u) {
    vis[u]=1;
    for (int &v : graph[u]) {
        if (vis[v]) continue;
        tree[u].push_back(v);
        tree[v].push_back(u);
        dfs(v);
    }
}
pii dfs2(int u, int p) {
    pii ans = {u, 0};
    for (int& v : tree[u]) {
        if (v == p) continue;
        pii cur = dfs2(v, u);
        if (cur.second+1 > ans.second) ans = {cur.first, cur.second+1};
    }
    return ans;
}
void dfs3(int u, int p) {
    height[u]=0;
    for(int& v : tree[u]) {
        if (v == p) continue;
        dfs3(v, u);
        height[u] = max(height[u], height[v]+1);
    }
}
void clean() {
    for (int i=0; i<n; i++) {
        graph[i].clear();
        tree[i].clear();
        vis[i]=0, height[i]=0, dis[i]=maxn;
    }
    ans.clear();
}
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);
        }
        dfs(0);
        pii node = dfs2(0, 0);
        int a=node.first;
        limit = ceil(sqrt(n));
        dfs3(a, a);

        for (int i=0; i<n; i++) {
            dis[i]=maxn;
            if (height[i]>0 && height[i]%limit == 0) ans.push_back(i);
        }
        queue<int> q;
        if (ans.empty()) ans.push_back(a);
        sort(ans.begin(), ans.end());
        ans.resize(unique(ans.begin(), ans.end()) - ans.begin());
        assert(ans.size()<= limit);

        for(int& i : ans) 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);
                }
            }
        }
        bool ok=1;
        for (int i=0; i<n; i++) ok &= dis[i]<=limit;
        if (ok) {
            cout << ans.size() << endl;
            for(int& i : ans) cout << i+1 << ' ';
            cout << endl;
        } else cout << -1 << endl;
        clean();
    }
    return 0;
}