#include <bits/stdc++.h>
#define fr first
#define sc second
#define all(a) (a).begin(), (a).end()
#define unique(a) a.resize(unique(a.begin(), a.end()) - a.begin())

using namespace std;

#ifdef ONPC
mt19937 rnd(223);
#else
mt19937 rnd(chrono::high_resolution_clock::now()
			.time_since_epoch().count());
#endif

#define TIME (clock() * 1.0 / CLOCKS_PER_SEC)

using ll = long long;
using ld = double;

const int maxn = 1e6 + 100, inf = 1e9 + 100;

int e[maxn][26];

int sz[maxn], ter[maxn];

int ts;

string ans;

int k;

bool go(int v, int cur) {
    cur += ter[v];
    for (int i = 0; i < 26; i++)
        if (e[v][i] != 0) {
            cur += min(1, sz[e[v][i]]);
        }
    if (cur >= k)
        return 1;
    for (int i = 0; i < 26; i++)
        if (e[v][i] != 0) {
            int w = sz[e[v][i]];
            cur -= min(1, w);
            if (go(e[v][i], cur)) {
                ans.push_back(i + 'a');
                return 1;
            }
            cur += w;
        }
    return 0;
}

void solve() {
    int n;
    cin >> n >> k;
    ts = 1;
    for (int i = 0; i < n; i++) {
        string s;
        cin >> s;
        int v = 0;
        sz[v]++;
        for (char c : s) {
            c -= 'a';
            if (e[v][c] == 0)
                e[v][c] = ts++;
            v = e[v][c];
            sz[v]++;
        }
        ter[v]++;
    }
    ans = "";
    assert(go(0, 0));
    reverse(all(ans));
    if (ans == "")
        ans = "EMPTY";
    cout << ans << '\n';
    // todo clear
    for (int i = 0; i < ts; i++) {
        sz[i] = 0;
        ter[i] = 0;
        fill(e[i], e[i] + 26, 0);
    }
}

int main() {
#ifdef ONPC
    freopen("../a.in", "r", stdin);
//    freopen("../a.out", "w", stdout);
#endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout << fixed;
    cout.precision(20);
    int ts;
    cin >> ts;
    while (ts--)
        solve();
    cerr << "\n\nConsumed " << TIME << endl;
}