#include <bits/stdc++.h>

using namespace std;
using ll = long long;

#define MAX(a, b) (a) = max((a), (b))
#define MIN(a, b) (a) = min((a), (b))
#define all(a) (a).begin(), (a).end()
#define sortedpair(a, b) {min((a), (b)), max((a), (b))}

const ll MOD = 1e9+7;

const int MAXN = 100'001;

struct E {
    int u, v, w;

    E(int a, int b, int c) : u(a), v(b), w(c) {}
};

int par[MAXN], ans[MAXN];
vector<int> g[MAXN];

int get(int u) {
    if (par[u] == u) return u;
    return par[u]=get(par[u]);
}

bool unio(int u, int v) {
    u = get(u); v=get(v);
    if (u == v) return false;
    par[u]=v;
    return true;
}

void dfs(int u, int p = -1, int d = 0) {
    ans[u] = d&1;
    for (int v : g[u]) {
        if (v != p) dfs(v, u, d+1);
    }
}

void solve(){
    int n, m; cin>>n>>m;
    iota(par, par+n+1, 0);
    map<pair<int, int>, int> mp;
    vector<pair<int, int>> a;
    for (int i = 0; i < m; i++) {
        int u, v; cin >> u >> v;
        a.emplace_back(u, v);
        if (u > v) swap(u, v);
        mp[{u, v}]++;
    }
    for (int i = 1; i <= n; i++) {
        g[i].clear();
    }

    vector<E> edges;
    for (auto [e, x] : mp) {
        edges.emplace_back(e.first, e.second, x);
    }
    sort(edges.begin(), edges.end(), [](auto x, auto y){
        return x.w > y.w;
    });
    for (E e : edges) {
        if (unio(e.u, e.v)) {
            g[e.u].emplace_back(e.v);
            g[e.v].emplace_back(e.u);
        }
    }
    dfs(1);
    int ok = 0;
    for (auto [u, v] : a) {
        if (ans[u] == ans[v]) ok++;
    }
    if (ok > m/2) {
        cout << "-1\n";
        return;
    }
    int cnt = count(ans+1, ans+n+1, 1);
    cout << cnt << "\n";
    for (int i = 1; i <= n; i++) {
        if (ans[i]) cout << i << " ";
    }
    cout << "\n";
}

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    int _t = 1;
    cin >> _t;
    while (_t--) {
        solve();
    }
    return 0;
}