// Problem: P9694 [GDCPC2023] New but Nostalgic Problem
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P9694
// Memory Limit: 1 MB
// Time Limit: 2000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include <bits/stdc++.h>
#define pb emplace_back
#define fst first
#define scd second
#define mkp make_pair
#define mems(a, x) memset((a), (x), sizeof(a))

using namespace std;
typedef long long ll;
typedef double db;
typedef unsigned long long ull;
typedef long double ldb;
typedef pair<ll, ll> pii;

const int maxn = 1000100;

int n, m, f[99], g[99];
char s[maxn];

struct Trie {
	int ch[maxn][26], ntot, sz[maxn];
	
	inline void init() {
		for (int i = 0; i <= ntot; ++i) {
			sz[i] = 0;
			for (int j = 0; j < 26; ++j) {
				ch[i][j] = 0;
			}
		}
		ntot = 0;
	}
	
	inline void insert(char *s) {
		int p = 0;
		for (int i = 0; s[i]; ++i) {
			if (!ch[p][s[i] - 'a']) {
				ch[p][s[i] - 'a'] = ++ntot;
			}
			p = ch[p][s[i] - 'a'];
			++sz[p];
		}
	}
} T;

void solve() {
	scanf("%d%d", &n, &m);
	T.init();
	for (int i = 1; i <= n; ++i) {
		scanf("%s", s);
		T.insert(s);
	}
	string ans = "";
	int p = 0;
	while (1) {
		for (int i = 0; i < 26; ++i) {
			g[i] = (i ? g[i - 1] : 0) + T.sz[T.ch[p][i]];
		}
		f[26] = (p ? T.sz[p] - g[25] : 0);
		for (int i = 25; ~i; --i) {
			f[i] = f[i + 1] + (T.ch[p][i] ? 1 : 0);
		}
		if (f[0] >= m) {
			break;
		}
		for (int i = 0; i < 26; ++i) {
			if (!T.ch[p][i]) {
				continue;
			}
			if (T.sz[T.ch[p][i]] >= 2 && T.sz[T.ch[p][i]] + (i ? g[i - 1] : 0) + f[i + 1] >= m) {
				m -= (i ? g[i - 1] : 0) + f[i + 1];
				m = max(m, 2);
				ans += char('a' + i);
				p = T.ch[p][i];
				break;
			}
		}
	}
	printf("%s\n", ans.size() ? ans.c_str() : "EMPTY");
}

int main() {
	int T = 1;
	scanf("%d", &T);
	while (T--) {
		solve();
	}
	return 0;
}