#include<bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<unordered_map>
#include<vector>
#include<bitset>
#include<queue>
#include<set>
#include<map>
#include<ctime>
#include<random>
#include<numeric>
using namespace std;
#define int long long
#define ll long long
#define ull unsigned long long
#define lc (x<<1)
#define rc (x<<1|1)
#define pii pair<int,int>
#define mkp make_pair
#define fi first
#define se second
const int Mx=1000005,p=998244353;
int read(){
	char ch=getchar();
	int Alice=0,Aya=1;
	while(ch<'0'||ch>'9'){
		if(ch=='-') Aya=-Aya;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9')
		Alice=(Alice<<3)+(Alice<<1)+(ch^48),ch=getchar();
	return (Aya==1?Alice:-Alice);
}
int n,k;
int No_;
int tr[Mx][26];
int cnt[Mx];
int ed[Mx];
bool done[Mx];
void Clear(){
	for(int i=1;i<=No_;i++){
		for(int j=0;j<26;j++) tr[i][j]=0;
		cnt[i]=0;
		done[i]=0;
		ed[i]=0;
	}
}
void Insert(string &s){
	int id=1;
	for(char c:s){
		int op=c-'a';
		if(!tr[id][op]) tr[id][op]=++No_;
		//cout<<id<<" "<<c<<" ";
		id=tr[id][op];
		//cout<<id<<endl;
		cnt[id]++;
	}
	ed[id]++;
}
string ans;
bool ok=0;
void Exit(){
	if(ans.empty()) puts("EMPTY");
	else puts(ans.c_str());
	Clear();
	ok=1;
}
void Delete(int u){
	//cout<<"BAN "<<u<<endl;
	cnt[u]--;
	done[u]=1;
	if(ed[u]){
		ed[u]--;
		return;
	}
	for(int op=0;op<26;op++){
		int v=tr[u][op];
		if(!v) continue;
		if(cnt[v]){
			Delete(v);
			return;
		}
	}
}
void dfs(int u){
	//cout<<u<<endl;
	if(ed[u]){
		k-=ed[u];
		cnt[u]-=ed[u];
		ed[u]=0;
		//cout<<"! "<<k<<endl;
		if(!ok&&k<=0){
			Exit();
			return;
		}
	}
	for(int op=0;op<26;op++){
		int v=tr[u][op];
		if(!v) continue;
		//cout<<u<<" -> "<<v<<" : "<<cnt[v]<<" "<<done[v]<<endl;
		if(cnt[v]&&!done[v]){
			Delete(v);
			//cout<<"NOW: "<<ans<<endl;
			k--;
			if(!ok&&k<=0){
				Exit();
				return;
			}
		}
		if(ok) return;
	}
	for(int op=0;op<26;op++){
		int v=tr[u][op];
		if(!v) continue;
		ans+='a'+op;
		if(cnt[v]) dfs(v);
		ans.pop_back();
		if(ok) return;
	}
}
void Solve(){
	n=read(),k=read();
	No_=1;
	ok=0;
	ans.clear();
	for(int i=1;i<=n;i++){
		string s;
		cin>>s;
		Insert(s);
	}
	dfs(1);
}
signed main(){
	int T=read();
	while(T--) Solve();
	return 0;
}
/*
Hello!!
Sample:
1
5 5
aaaba
acc
bbbab
a
bbb

200
5 3
gdcpc
gdcpcpcp
suasua
suas
sususua
3 3
a
b
c
1
4 4
baba
b
aba
b
-------------------
*/