#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("avx2")
//#pragma GCC target("sse4")
#include <bits/stdc++.h>
using namespace std;

//#include<ext/pb_ds/assoc_container.hpp>
//#include<ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;

//template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define DEBUG
#define x first
#define y second
#define pb push_back
#define all(a) a.begin(),a.end()
#define uid(a, b) uniform_int_distribution<int>(a, b)(rng)

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<ll> vl;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<pii> vii;
typedef vector<pll> vll;

template<class T> bool umin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool umax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937 rnd(time(nullptr));

void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(ll x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(ull x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(ld x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef DEBUG
#define dbg(x...) cerr << __func__<<":"<<__LINE__<<" [" << #x << "] = ["; _print(x); cerr << endl;
#else
#define dbg(x...)
#endif

const int N = 2e5 + 5, inf = 1e9;

vi g[N];
int dist[N], n, sq;
int pr[N];
bool used[N];

void bfs(int *d, int s) {
    for (int i = 1; i <= n; ++i) {
        used[i] = false;
        d[i] = inf;
    }
    used[s] = true;
    d[s] = 0;
    pr[s] = -1;
    queue<int> q;
    q.push(s);
    while (!q.empty()) {
        int cur = q.front();
        q.pop();
        for (int to : g[cur]) {
            if (!used[to]) {
                used[to] = true;
                dist[to] = dist[cur] + 1;
                pr[to] = cur;
                q.push(to);
            }
        }
    }
}


void solve() {
    int m; cin >> n >> m;
    sq = 1;
    while (sq * sq < n) sq++;
    for (int i = 1; i <= n; ++i) {
        g[i].clear();
    }
    for (int i = 1; i <= m; ++i) {
        int u, v; cin >> u >> v;
        g[u].pb(v), g[v].pb(u);
    }
    bfs(dist, 1);
    int st = 1;
    for (int i = 1; i <= n; ++i) {
        if (dist[i] > dist[st]) {
            st = i;
        }
    }
    // cout << "diam: " << st << " ";
    bfs(dist, st);
    st = 1;
    for (int i = 1; i <= n; ++i) {
        if (dist[i] > dist[st]) {
            st = i;
        }
    }
    // cout << st << " ";
    int cnt = dist[st];
    while (dist[st] != cnt / 2) {
        st = pr[st];
    }
    // cout << st << "\n";
    for (int i = 1; i <= n; ++i) {
        used[i] = false;
        dist[i] = inf;
    }
    used[st] = true;
    dist[st] = 0;
    queue<int> q;
    q.push(st);
    vi ans;
    ans.pb(st);
    while (!q.empty()) {
        int cur = q.front();
        q.pop();
        for (int to : g[cur]) {
            if (!used[to]) {
                used[to] = true;
                dist[to] = dist[cur] + 1;
                if (dist[to] % (sq + 1) == 0) {
                    ans.pb(to);
                }
                q.push(to);
            }
        }
    }
    if (ans.size() > sq) {
        cout << "-1\n";
    }
    else {
        cout << ans.size() << "\n";
        for (int v : ans) {
            cout << v << " ";
        }
        cout << "\n";
    }
}


int main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    int t; cin >> t;
    while (t--) solve();
}