#include<bits/stdc++.h>
using namespace std;
#define int long long
#define INF 0x3f3f3f3f3f3f3f3f
#define pii pair<int,int>
#define fi first
#define se second
#define all(_a) _a.begin(), _a.end()

#define rep(i,p,q) for (int i=(p);i<=(q);i++)
const int N=1000010;
int son[N][30],cnt[N],idx,a[N];
int n,m,flag,t;string res;

void insert(string s){
    int p=0;
    for(int i=0;i<s.size();i++){
        int u=s[i]-'a'+1;
        if(!son[p][u]) son[p][u]=++idx,cnt[p]++;
        p=son[p][u];
    }
    son[p][0]=1,cnt[p]++;
}
void print(int x) {flag=1; if (!x) puts("EMPTY"); else {rep(i,1,x) putchar(a[i]+96); puts("");}}
void dfs(int p,int x){
    if (flag) return;
    //printf("%d %d\n",p,cnt[p]);
    if (t>=m) return print(x-2),void();
    t+=cnt[p]-1;
    if (t>=m) return print(x-1),void();
    rep(i,1,26) if (son[p][i]) a[x]=i,dfs(son[p][i],x+1);
}
/*
int b[N];
void dfs1(int x,int p) {
    int q=0;
    rep(i,1,26) if (son[x][i]) q++,b[p]=i,dfs1(son[x][i],p+1);
    //if (son[x][0]) {rep(i,1,p-1) putchar(b[i]+96); puts("");}
    //if (q>1) cout<<cnt[x]<<" "<<q<<"\n";
}*/
int last;
void solve(){
    for(int i=0;i<=last+2;i++){
        cnt[i]=0;
        for(int j=0;j<26;j++)son[i][j]=0;
    }
    idx=0;
    cin>>n>>m;
    res.clear();
    int num=0;
    last=0;
    flag=0;
    t=1;
    for(int i=1;i<=n;i++){
        string s;
        cin>>s;
        last+=s.size();
        insert(s);
    }
  //  dfs1(0,1);
    dfs(0,1);
}
signed main(){
   ios::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);
    cout << setiosflags(ios::fixed) << setprecision(15);
    int T;
    T=1;
    cin>>T;
    while(T--)solve();
    return 0;
}