#include <bits/stdc++.h>
using namespace std;
const int N = 2e6 + 10;

struct Tire {
    vector<array<int, 26>>ch;
    vector<int>siz, cnt;
    int idx;
    Tire() {
        idx = 0;
        ch.assign(N, {});
        cnt.resize(N);
        siz.resize(N);
        // pos.resize(N);
    }
    Tire(int n) {
        idx = 0;
        ch.assign(n + 1, {});
        cnt.resize(n + 1);
        siz.resize(n + 1);
        // pos.resize(n + 1);
    }
    void insert(string& s, int now) {
        int p = 0;
        for (auto x : s) {
            int c = x - 'a';
            if (!ch[p][c])ch[p][c] = ++idx;
            p = ch[p][c];
            siz[p]++;
        }
        // pos[p] = now;
        cnt[p]++;
    }
    int query(string& s) {
        int p = 0;
        for (auto x : s) {
            int c = x - 'a';
            if (!ch[p][c])return 0;
            p = ch[p][c];
        }
        return cnt[p];
    }
    void solve(int k) {
        int p = 0;
        // auto dfs = [&](auto&& ff, int p)->void {
        while (1) {
            int sum = cnt[p];
            for (int i = 0;i < 26;i++) {
                if (siz[ch[p][i]])sum++;
            }
            if (sum >= k) {
                if (p == 0) {
                    cout << "EMPTY\n";return;
                }
                return;
            }
            for (int i = 0;i < 26;i++) {
                if (siz[ch[p][i]]) {
                    sum += siz[ch[p][i]] - 1;
                    if (sum >= k) {
                        k -= sum - siz[ch[p][i]];
                        cout << (char)(i + 'a');
                        p = ch[p][i];
                        break;
                        // ff(ff, ch[p][i]);
                    }
                }
            }
        };
        // dfs(dfs, 0);
        // cout << "\n";
    }
};

void solve() {
    int n, k;cin >> n >> k;

    vector<string>s(n + 1);
    int mx = 0;
    for (int i = 1;i <= n;i++) {
        cin >> s[i];
        mx = max(mx, (int)s[i].size());
    }
    Tire tr(mx + 10);
    for (int i = 1;i <= n;i++) {
        tr.insert(s[i], i);
    }
    tr.solve(k);
    cout << "\n";
}

signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int T = 1;
#ifdef LOCAL
    freopen("data.in", "r", stdin);
    freopen("data.out", "w", stdout);
#endif
    cin >> T;
    while (T--)
        solve();
    return 0;
}