#include "bits/stdc++.h"
using namespace std;

using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pll = pair<ll, ll>;

struct Trie {
	int here = 0;
	int subtr = 0;
	vector<pair<char, int>> trans;
};

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

	vector<string> rs(n);
	int sum = 0;
	for (int i = 0; i < n; i++) {
		cin >> rs[i];
		sum += rs[i].size();
	}

	vector<Trie> S(sum + 5);

	Trie& root = S[0];

	int cnt = 1;

	auto ins = [&](auto&& ins, Trie& tr, string &s, int pos) -> void {
		++tr.subtr;
		if (pos == (int)s.size()) {
			++tr.here;
			return;
		}
		bool fnd = false;
		for (auto &[ch, pnt]: tr.trans) {
			if (ch == s[pos]) {
				fnd = true;
				ins(ins, S[pnt], s, pos + 1);
			}
		}
		if (!fnd) {
			++cnt;
			tr.trans.push_back({s[pos], cnt - 1});
			ins(ins, S[cnt - 1], s, pos + 1);
		}
	};

	for (int i = 0; i < n; i++) {
		ins(ins, root, rs[i], 0);
	}

	string ans = "";
	auto que = [&](auto &&que, Trie &tr, int k) -> void {
		k -= tr.here;
		int cnttr = tr.trans.size();
		if (k <= cnttr) {
			return;
		}
		sort(begin(tr.trans), end(tr.trans));
		int i = 0;
		for (auto &[ch, pnt]: tr.trans) {
			++i;
			if (cnttr - i + S[pnt].subtr >= k) {
				ans += ch;
				que(que, S[pnt], k - (cnttr - i));
				return;
			}
			else {
				k -= S[pnt].subtr;
			}
		}
	};
	que(que, root, k);

	if (ans.size() == 0) {
		cout << "EMPTY\n";
	}
	else {
		cout << ans << "\n";
	}
}

int main() {
	cin.tie(0)->sync_with_stdio(0);
	int t;
	cin >> t;
	while (t--) {
		solve();
	}	
}