#include <bits/stdc++.h>
#define rep(i,l,r) for(int i=l; i<=r; i++)
#define IOS {cin.tie(0);cout.tie(0);ios::sync_with_stdio(0);}
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

const int N = 1000010;

char s[N];
int ch[N][26], tag[N], tot, ans, n, k, top;
char st[N];

bool dfs(int u) {
    ans += tag[u];
    rep (j, 0, 25) {
        if (ch[u][j]) ans++;
    }
    if (ans >= k) {
        if (top) rep(i, 1, top) cout << st[i];
        else cout <<"EMPTY";

        cout <<'\n';
        return true;
    }
    rep (j, 0, 25) {
        if (ch[u][j]) {
            st[++top] = j + 'a';
            ans--;
            if (dfs(ch[u][j])) return true;
            // ans++;
            top--;
        }
    }
    return false;
}

int main() {
    int T; scanf("%d", &T);
    while (T--) {
        scanf("%d%d", &n, &k);
        rep (i, 1, n) {
            scanf("%s", s+1);
            int len = strlen(s+1);
            int now = 0;
            rep (j, 1, len) {
                int c = s[j] - 'a';
                if (!ch[now][c]) ch[now][c] = ++tot;
                now = ch[now][c];
            }
            tag[now]++;
        }
        dfs(0);
        rep (i, 0, tot) {
            memset(ch[i], 0, sizeof(ch[i]));
            tag[i] = st[i] = 0;
        }
        tot = ans = top = 0;
    }

    return 0;
}