#include <bits/stdc++.h>
using namespace std;
using LL = long long;
constexpr int N = 1E6 + 5;

int cnt;
int tr[N][26] , pre[N] , sz[N];

void solve() {
	
	int n,k;
	cin >> n >> k;

	auto Newnode = [&] (void) {
		++cnt;
		pre[cnt] = sz[cnt] = 0;
		for (int j = 0 ; j < 26 ; ++j) tr[cnt][j] = 0;
		return cnt;
	};

	cnt = 0;
	Newnode();

	auto add = [&] (string s) {
		int m = s.size() , now = 1;
		pre[now] ++;
		for (int i = 0 ; i < m ; ++i) {
			int x = s[i] - 'a';
			if (!tr[now][x]) tr[now][x] = Newnode();
			now = tr[now][x];
			pre[now] += 1;
		}
		sz[now] += 1;
	};

	for (int i = 0 ; i < n ; ++i) {
		string s;
		cin >> s;
		add(s);
	}

	int now = 1;
	while (true) {
		int t = sz[now];
		for (int i = 0 ; i < 26 ; ++i) {
			if (pre[tr[now][i]]) t ++;
		}
		if (t >= k) {
			if (now == 1) cout << "EMPTY";
			cout << "\n";
			return;
		}
		for (int i = 0 ; i < 26 ; ++i) {
			if (pre[tr[now][i]]) {
				t = t - 1 + pre[tr[now][i]];
				if (t >= k) {
					k -= (t - pre[tr[now][i]]);
					now = tr[now][i];
					cout << char(i + 'a');
					break;
				}
			}
		}
	}
}

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

	int t = 1;
	cin >> t;
	while (t--) {
		solve();
	}

	return 0;
}