#include<bits/stdc++.h>
# define rep(i, j, k) for (int i = j; i <= k; ++i)
# define ed(id) tr[id].ed
# define to(id, x) tr[id].to[x]
# define bk(id, x) tr[id].bk[x]
# define cnt(id) tr[id].cnt
using namespace std;

struct node
{
	vector < int > ed;
	int to[26], bk[27], cnt;
} tr[2000005];
int T, n, k, nu;
string s[1000005];
int stk[1000005], r;
void insert(int x)
{
	int u = 1;
	for (int i = 0; i < s[x].size(); ++i) {++cnt(u); if (!to(u, s[x][i] - 'a')) to(u, s[x][i] - 'a') = ++nu; u = to(u, s[x][i] - 'a');}
	ed(u).push_back(x); ++cnt(u);
}
bool dfs(int u, int num)
{
	num += ed(u).size();
	for (int i = 25; i >= 0; --i) bk(u, i) = bk(u, i + 1) + !!to(u, i);
	if (bk(u, 0) + num >= k)
	{
		rep(i, 1, r) cout << (char) (stk[i] + 'a');
		cout << '\n';
		return true;
	}
	int al = 0;
	rep(i, 0, 25)
	{
		stk[++r] = i;
		if (to(u, i) && dfs(to(u, i), num + al + bk(u, i + 1))) return true;
		--r; al += cnt(to(u, i));
	}
	return false;
}
signed main()
{
	ios::sync_with_stdio(false); cin.tie(0), cout.tie(0);
	cin >> T;
	while (T--)
	{
		r = 0;
		rep(i, 1, nu) {ed(i).clear(); cnt(i) = 0; rep(j, 0, 25) to(i, j) = bk(i, j) = 0;} nu = 1;
		cin >> n >> k;
		rep(i, 1, n) cin >> s[i], insert(i);
		int ppp = 0;
		rep(i, 0, 25) ppp += !!to(1, i);
		if (ppp >= k) {cout << "EMPTY\n"; continue;}
		dfs(1, 0);
	}
	return 0;
}