#include<bits/stdc++.h>
using namespace std;
#define LL long long
#define ld long double
#define i64 __int128
#define PII pair<int,int>
#define endl '\n'
#define pb push_back
#define fi first
#define se second
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
//cout<<setiosflags(ios::fixed)<<setprecision(K);
const int dx[4] = {1,-1,0,0};
const int dy[4] = {0,0,1,-1};
const double eps = 1e-9; 
const double PI = acos(-1.0);
const int INF = 1e9 + 7;
const int mod = 998244353;
const int N = 1e6 + 10,M = N << 1;

struct Trie{
    vector<vector<int>> ch;
    vector<int> cnt ,sz;
    int idx = 0;
    int n;
    void init(int n){
        this -> n = n;
        cnt.resize(n);
        sz.resize(n);
        ch.resize(n,vector<int>(26));
    }
    void insert(string s){
        LL u = 0;
        for (int i = 0; i < s.size(); i ++ ){
            LL v = s[i] - 'a';
            if (!ch[u][v]) ch[u][v] = ++ idx;
            u = ch[u][v];
        	cnt[u] ++;
        }
        sz[u] ++;
    }
    string query(int m){
        int u = 0;
        string ans = "";
        while(1){
        	int num = sz[u];
        	// int num = 0;
        	for(int i = 0;i <= 25;i ++){
        		if(ch[u][i] != 0) num ++;
        	}
        	if(num >= m) return ans;
        	int ok = 0;
        	for(int i = 0;i <= 25;i ++){
        		if(ch[u][i] == 0) continue;
        		num += max(0,cnt[ch[u][i]] - 1);
        		if(num >= m){
        			ans.pb(char(i + 'a'));
        			ok = 1;
        			num -= (cnt[ch[u][i]] - 1);
        			m = 1 + (m - num);
        			u = ch[u][i];
        			break;
        		}
        		if(ok == 1) break;
        	}
        	if(ok == 1) continue;
        }
    }
};

void solve(){
    int n,m;
    cin >> n >> m;
    int len = 0;
    vector<string> a(n + 1);
    for(int i = 1;i <= n;i ++){
    	cin >> a[i];
    	int siz = a[i].size();
    	len += siz;
    }
    sort(a.begin() + 1,a.end());
    Trie tr;
    tr.init(len + 1);
    for(int i = 1;i <= n;i ++){
    	tr.insert(a[i]);
    }
    string res = tr.query(m);
    if(res == "") cout << "EMPTY" << endl;
    else cout << res <<endl;
}

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