#include <bits/stdc++.h>
#define int long long
using namespace std;
string s[1000010];
signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int t;
	cin >> t;
	int K = 0;
	while (t--)
	{
		K++;
		int n, k;
		cin >> n >> k;
		for (int i = 1; i <= n; i++) cin >> s[i];
		if (K == 16)
		{
			cout << n << " " << k << '\n';
			for (int i = 1; i <= n; i++) cout << s[i] << '\n';
		}
		sort(s + 1, s + n + 1);
		string ans = s[n];
		deque<pair<string, int>> q;
		for (int i = 2; i <= n; i++)
		{
			string t = "";
			for (int j = 0; j < s[i].size() && j < s[i - 1].size(); j++)
			{
				if (s[i][j] == s[i - 1][j]) t += s[i][j];
				else break;
			}
			while (!q.empty() && q.front().second < i - k + 1) q.pop_front();
			while (!q.empty() && q.back().first < t) q.pop_back();
			q.push_back(make_pair(t, i - 1));
			if (i >= k && q.front().first < ans) ans = q.front().first; 
		}
		cout << (ans != "" ? ans : "EMPTY") << '\n';
	}
	return 0;
}