#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
#define op(x) ((x&1)?x+1:x-1)
#define odd(x) (x&1)
#define even(x) (!odd(x))
#define lc(x) (x<<1)
#define rc(x) (lc(x)|1)
#define lowbit(x) (x&-x)
#define mp(x,y) make_pair(x,y)
typedef long long ll;
typedef unsigned long long ull;
typedef double db;
using namespace std;
const int MAXN = 1e6+10,M = 26;

int T,n,k;

int ch[MAXN][M],cnt[MAXN],sz[MAXN],tot;
char ans[MAXN];

int now;

int out(){
	if(!now)cout<<"EMPTY\n";
	else{
		cout<<(ans)<<"\n";
		rep(i,0,now-1)ans[i]=0;
	}
	return 1;
}

int dfs(int u,int num){
	num += cnt[u];
	rep(i,0,M-1)if(ch[u][i])num++;
	if(num >= k)return out();

	int v=0;
	rep(i,0,M-1)if(v=ch[u][i]){
		ans[now++]='a'+i;
		if(dfs(v,num-1))return 1;
		ans[now--]=0;
		num+=(sz[v]-1);
	}

	return 0;
}

void solve(){
	rep(i,0,tot){
		cnt[i]=sz[i]=0;
		memset(ch[i],0,sizeof ch[i]);
	} tot = 0;
	//
	cin>>n>>k;
	rep(i,1,n){
		string s; cin>>s;
		int len = s.length();
		int u=0; 
		rep(j,0,len-1){
			int c=s[j]-'a';
			if(!ch[u][c])ch[u][c]=++tot;
			u=ch[u][c];
			sz[u]++;
		}	
		cnt[u]++;
	}

	now = 0;
	assert(dfs(0,0));
}

int main(){
	ios::sync_with_stdio(false); cin.tie(0);

	cin>>T;
	while(T--)solve();

    return 0;
}