#include<bits/stdc++.h>                               
#define rep(i,p,q) for (ll i=(p);i<=(q);i++)          
#define pre(i,p,q) for (ll i=(p);i>=(q);i--)          
#define ll long long                                  
using namespace std;                                  
const int N=1e6+5; int n,m,cnt,flag,a[N],ed[N],son[N][30],ans[N];
void print(int x) {flag=1; if (!x) puts("EMPTY"); else {rep(i,1,x) putchar(ans[i]+96); puts("");}}
void dfs(int x,int s,int p) {
	if (flag) return;
	int t=0;
	rep(i,1,26) if (son[x][i]) t++;
	if (s+t+ed[x]>=m) return print(p-1);
	rep(i,1,26) if (son[x][i]) ans[p]=i,dfs(son[x][i],s+t-1+ed[x],p+1),t+=a[son[x][i]]-1;
}
void solve() {
	int x,l; char st[N];
	scanf("%d%d",&n,&m);
	rep(i,1,n) {
		scanf("%s",st+1),l=strlen(st+1);
		x=0;
		rep(j,1,l) {
			if (!son[x][st[j]-96]) son[x][st[j]-96]=++cnt;
			a[x=son[x][st[j]-96]]++;
		}
		ed[x]++;
	}
	dfs(0,0,1);
	rep(i,0,cnt) memset(son[i],0,sizeof(son[i])),a[i]=ed[i]=0;
	cnt=flag=0;
}                                                     
int main() {int T; for (scanf("%d",&T);T--;) solve();}