#include <cstdio>
#include <cstring>
#include <string>
#include <iostream>

using namespace std;

const int E = 26, S = 1000000;
int n, num;

struct Tr
{
	int son[E + 5];
	int siz;
	int end;
}tr[S + 5];
int cntn = 0;

void ist(string s)
{
	int len = s.size();
	int p = 0;
	for(int i = 0; i < len; i++)
	{
		if(!tr[p].son[s[i] - 'a'])
		{
			tr[p].son[s[i] - 'a'] = ++cntn;
		}
		p = tr[p].son[s[i] - 'a'];
		tr[p].siz++;
		if(i == len - 1)
		{
			tr[p].end++;
		}
	}
}

void init()
{
	scanf("%d %d", &n, &num);
	for(int i = 1; i <= n; i++)
	{
		string s;
		cin >> s;
		ist(s);
	}
}

bool flag = 0;
void dfs(int u, int x)
{
	x -= tr[u].end;
	for(int i = 0; i < E; i++)
	{
		if(tr[u].son[i] && x)
		{
			x--;
		}
	}
	if(x == 0)
	{
		return;
	}
	for(int i = 0; i < E; i++)
	{
		if(!tr[u].son[i])
		{
			continue;
		}
		if(x <= tr[tr[u].son[i]].siz - 1)
		{
			printf("%c", i + 'a');
			flag = 1;
			dfs(tr[u].son[i], x + 1);
			return;
		}
		else
		{
			x -= tr[tr[u].son[i]].siz - 1;
		}
	}
}

void solve()
{
	dfs(0, num);
	if(!flag)
	{
		printf("EMPTY");
	}
	printf("\n");
}

void clr()
{
	for(int i = 0; i <= cntn; i++)
	{
		tr[i].siz = tr[i].end = 0;
		for(int j = 0; j < E; j++)
		{
			tr[i].son[j] = 0;
		}
	}
	cntn = 0;
	flag = 0;
}

int main()
{
	int T;
	scanf("%d", &T);
	while(T--)
	{
		init();
		solve();
		clr();
	}
	return 0;
}