/*
[templates]: 
duipai
spjdp
compre
addhis
*/
#include<bits/stdc++.h>
using namespace std;
#define int long long
//use ll instead of int.
#define f(i, a, b) for(int i = (a); i <= (b); i++)
#define cl(i, n) i.clear(),i.resize(n);
#define endl '\n'
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int inf = 1e9;
//#define cerr if(false)cerr
//#define freopen if(false)freopen
mt19937 rng(time(0)); 
int rnd(int l, int r) {return rng() % (r-l+1) + l; }
#define watch(x) cerr  << (#x) << ' '<<'i'<<'s'<<' ' << x << endl
void pofe(int number, int bitnum) {
    string s; f(i, 0, bitnum) {s += char(number & 1) + '0'; number >>= 1; } 
    reverse(s.begin(), s.end()); cerr << s << endl; 
    return;
}
template <typename TYP> void cmax(TYP &x, TYP y) {if(x < y) x = y;}
template <typename TYP> void cmin(TYP &x, TYP y) {if(x > y) x = y;}
//调不出来给我对拍！
//use std::array.
string s[1000010];
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
    //freopen();
    //freopen();
    //time_t start = clock();
    //think twice,code once.
    //think once,debug forever.
    int T; cin >> T;
    while(T--) {
        int n,k;cin>>n>>k;
        f(i,1,n)cin>>s[i];
        sort(s+1,s+n+1); 
      //  f(i,1,n)cerr<<s[i]<<endl; 
        string u="";int d=0;int l=1,r=n;
        while(1) { 
        //    cerr<<d<<" "<<l<<" "<<r<<" "<<k<<endl; 
            int lst=r;int cnt=0; char ch; int nowl=0,nowr=0;
            for(int i=r;i>=l;i--) {
                if(i==l||((int)s[i].size()>d && (int)s[i-1].size()<=d)||((int)s[i].size()>d && (int)s[i-1].size()>d && s[i][d] != s[i-1][d])) {
                    if((lst-l+1) + cnt >= k) {
                        ch = ((int)s[i].size()>d?s[i][d]:' '),nowl=i,nowr=lst;
                    }
                    else {break;}cnt++; lst = i - 1; 
                    if(i==l) break;
                    
                    
                }
            }
          //  cerr<<nowl<<" "<<nowr<<"lst, cnt="<<lst<<" "<<cnt<<"ch= " << ch <<endl; 
            if(ch==' ' || cnt >= k) {break;}
            else {u += ch;k-=(nowr-l+1 - (nowr-nowl+1) + cnt - 1); l=nowl,r=nowr;d++; }
        }
        cout << (u==""?"EMPTY":u) << endl; 
    }
    //time_t finish = clock();
    //cout << "time used:" << (finish-start) * 1.0 / CLOCKS_PER_SEC <<"s"<< endl;
    return 0;
}