#include <bits/stdc++.h>

using namespace std;
#define IOS ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr)
#define rep(a, b, c) for(int (a)=(b);(a)<=(c);(a)++)
#define per(a, b, c) for(int (a)=(b);(a)>=(c);(a)--)
#define mset(var, val) memset(var,val,sizeof(var))
#define ll long long
#define int ll
#define fi first
#define se second
#define no "NO\n"
#define yes "YES\n"
#define pb push_back
#define endl "\n"
#define pii pair<int,int>
#define pll pair<ll,ll>
#define dbg(x...) do{cout<<#x<<" -> ";err(x);}while (0)

void err() { cout << '\n'; }

template<class T, class... Ts>
void err(T arg, Ts... args) {
    cout << arg << ' ';
    err(args...);
}

const int N = 2e5 + 5;
const int M = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3f;
const int P = 1e9 + 7;
const double eps = 1e-8;
const double pi = acos(-1.0);

int n, m, sqn, s, t, mid, cnt, fa[N], dep[N], mx_dp[N];
int tot=0, lnk[N], son[M<<1], nxt[M<<1];
bool f[N], vis[N];

void add(int x, int y) {
    son[++tot] = y, nxt[tot] = lnk[x], lnk[x] = tot;
}

void dfs1(int u) {
    for(int i = lnk[u]; i != -1; i = nxt[i]) {
        int v = son[i];
        if(vis[v]) continue;
        dep[v] = dep[u]+1, vis[v] = 1, fa[v] = u;
        dfs1(v);
    }
}

void dfs2(int u) {
    mx_dp[u] = 1;
    for(int i = lnk[u]; i != -1; i = nxt[i]) {
        int v = son[i];
        if(vis[v]) continue;
        vis[v] = 1;
        dfs2(v);
        mx_dp[u] = max(mx_dp[u], mx_dp[v]+1);
    }
    if(mx_dp[u] > sqn) {
        f[u]=1;
        cnt++;
    }
}

void solve() {
    cin >> n >> m;
    sqn = sqrt(n-1)+1, tot=0;
    for(int i = 1; i <= n; i++)
        f[i] = 0, lnk[i] = -1,vis[i]=0;
    for(int i = 1; i <= m; i++) {
    	int x, y;
        cin >> x >> y;
        add(x, y);
        add(y, x);
    }
    
    vis[1]=1, dep[1]=1, fa[1]=1;
    dfs1(1);

    s=1;
    for(int i = 1; i <= n; i++) {
        vis[i]=0;
        if(dep[i] > dep[s]) s = i;
        // printf("%d dep%d fa%d\n", i, dep[i], fa[i]);
    }
    
    vis[s]=1, dep[s]=1, fa[s]=1;
    dfs1(s);
    
    t=s;
    for(int i = 1; i <= n; i++) {
        vis[mid]=0;
        if(dep[i] > dep[t]) t = i;
        // printf("%d dep%d fa%d\n", i, dep[i], fa[i]);
    }
    mid = t;
    while(dep[mid] != dep[t]/2) mid = fa[mid];
    // printf("mid%d\n", mid);

    cnt=1, vis[mid] = 1, f[mid] = 1;
    dfs2(mid);

    if(cnt <= sqn) {
        printf("%d\n", cnt);
        for(int i = 1; i <= n; i++)
            if(f[i]) printf("%d ", i);
        printf("\n");
    }
    else printf("-1\n");
}

signed main() {
    IOS;
    int T = 1;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}