#include<bits/stdc++.h>
using namespace std;

typedef long long ll;

const int N = 1e6 + 10;

int n, K, idx;

struct Node {
    int cnt, ch[26];
}tr[N];

void insert(string s) {
    int cur = 0;
    for(auto c : s) {
        if(!tr[cur].ch[c - 'a']) {
            tr[cur].ch[c - 'a'] = ++ idx;
        }
        tr[tr[cur].ch[c - 'a']].cnt += 1;
        cur = tr[cur].ch[c - 'a'];
    }
}

void query(string &ans, int K) {
    tr[0].cnt = n;
    int cur = 0;
    while(true) {
        int cnt = 0;
        for(int i = 0; i < 26; i ++ ) {
            if(tr[cur].ch[i]) {
                cnt ++ ;
            }
        }

        bool ok = false;
        for(int i = 0; i < 26; i ++ ) {
            if(tr[cur].ch[i] && tr[tr[cur].ch[i]].cnt >= 2) {
                if(tr[tr[cur].ch[i]].cnt + cnt - 1 >= K) {
                    ans += (char)(i + 'a');
                    K -= cnt - 1, cur = tr[cur].ch[i];
                    ok = true;
                    break;
                } else {
                    cnt = cnt - 1 + tr[tr[cur].ch[i]].cnt;
                }
            }
        }

        if(!ok) break;
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int T;
    cin >> T;
    while(T -- ) {
        cin >> n >> K;
        for(int i = 0; i < n; i ++ ) {
            string s;
            cin >> s;
            insert(s);
        }

        string ans;
        query(ans, K);
        if(!ans.empty()) cout << ans << '\n';
        else cout << "EMPTY" << '\n';

        for(int i = 0; i <= idx; i ++ ) {
            tr[i].cnt = 0;
            for(int j = 0; j < 26; j ++ ) {
                tr[i].ch[j] = 0;
            }
        }
        idx = 0;
    }
    return 0;
}