#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const ll N = 2e5+7,mod=1e9+7,INF=1e18;
#define endl '\n'

void solve(){
	ll n, k;
	cin >> n >> k;
	vector<string> ve(n + 1);
	for(ll i = 1; i <= n; i++){
		string s; cin >> s;
		ve[i] = s;
	}
	
	sort(ve.begin() + 1, ve.end());
	
	auto find1 = [&](auto &&find1, ll l, ll r, ll dex, ll k1) -> string{
			
//		cout << l << " " << r << " " << dex << " " << k1 << endl;
			
		map<ll, ll> mp;
		ll sum = 0, cnt = 1;
		for(ll i = l; i <= r; i++){
			if((int)ve[i].size() <= dex){
				sum++;
				continue;
			} 
			mp[ve[i][dex] - 'a']++;
		}
		
		if(sum != 0){
			return find1(find1, l + sum, r, dex, k1 - sum);
		}
		
//		for(auto [x, y] : mp){
//			cout << x << " " << y << endl;
//		}
		
		ll num = mp.size();
		if(num >= k1) return ""; 
			
		for(ll i = 0; i < 26; i++){
			if(mp[i] == 0) continue;
			if(num - cnt + mp[i] + sum >= k1){
				ll c1 = k1 - num + cnt - sum;
				return (char)('a' + i) + find1(find1, l + sum, l + mp[i] - 1, dex + 1, c1);
			}else{
				cnt++;
				sum += mp[i];
			}	
		}
		return "";
	};
	
	string s = find1(find1, 1, n, 0, k);
	if(s == "") cout << "EMPTY" << endl;
	else cout << s << endl;
}

int main(){
	std::ios::sync_with_stdio(false);
	cin.tie(0);
	ll T=1;
	
	cin>>T;
	
	while(T--){
		solve();
	}
	return 0;
}


/*
1
4 4
abcd
amxy
aser
adfg
1
4 2
abcd
amxy
aser
adfg
1 
4 4
x
t
z
y
1
4 3
aaaaaaaaaaaaaaaaaaaaa
aaa
aaaaaaaaaaaaa
aaaaaaaa
*/