#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define x first
#define y second

using namespace std;

typedef long double ld;
typedef long long ll;

struct Node
{
	int next[26];
	int term, end;
};

Node bor[1000005];
int quant = 0;

int main()
{
 	//freopen("input.txt", "r", stdin);
 	//freopen("output.txt", "w", stdout);
	ios_base::sync_with_stdio(0); cin.tie(0);

	int test;
	cin >> test;

	int root = 1;
	for (int rep = 1; rep <= test; rep++)
	{
	 	for (int i = 1; i <= quant; i++)
	 	{
	 	 	for (int j = 0; j < 26; j++) bor[i].next[j] = 0;
	 	 	bor[i].term = 0, bor[i].end = 0;
	 	}
	 	quant = 1;
	 	
	 	int n, k;
	 	cin >> n >> k;

	 	for (int i = 1; i <= n; i++)
	 	{
	 	 	string s;
	 	 	cin >> s;

	 	 	int cur = root;
	 	 	for (char c : s)
	 	 	{
	 	 	 	int d = c - 'a';
	 	 	 	if (bor[cur].next[d] == 0) bor[cur].next[d] = ++quant;
	 	 	 	cur = bor[cur].next[d];
	 	 	 	bor[cur].term++;
	 	 	}
	 	 	bor[cur].end++;
	 	}

	 	string ans = "";
	 	int cur = root;
	 	while (true)
	 	{
	 	 	k -= bor[cur].end;

	 	 	int son = 0;
	 	 	for (int i = 0; i < 26; i++) son += (bor[cur].next[i] != 0);
	 	 	if (son >= k) break;

	 	 	for (int i = 0; i < 26; i++)
	 	 	{
	 	 	 	if (bor[cur].next[i] == 0) continue;
	 	 	 	son += bor[bor[cur].next[i]].term - 1;

	 	 	 	if (son >= k)
	 	 	 	{
	 	 	 	 	cur = bor[cur].next[i];
	 	 	 	 	ans += (char) ('a' + i);
	 	 	 	 	k -= (son - bor[cur].term);
	 	 	 	 	break;
	 	 	 	}
	 	 	}
	 	}

	 	if (ans.empty()) cout << "EMPTY\n";
	 	else cout << ans << '\n';
	}

	return 0;
}