#include <bits/stdc++.h>
using namespace std;

#define all(v) v.begin(), v.end()

using LL = long long;

int main() {
  cin.tie (nullptr) -> ios_base :: sync_with_stdio (false);

  int tests;
  cin >> tests;
  while (tests--) {
    int n, k;
    cin >> n >> k;
    vector <int> pref (n + 1);
    string text = "";
    for (int i = 1; i <= n; i++) {
      string s;
      cin >> s;
      text += s;
      pref[i] = s.size () + pref[i - 1];
    }
    vector <string> v;
    for (int i = 0; i + k + 1 <= n; i++) {
      string s = text.substr (pref[i], pref[i + k + 1] - pref[i]);
      v.push_back (s);
    }
    sort (all (v));
    sort (all (v), [&] (string a, string b) {
      if (a.size () == b.size ()) return a < b;
      return a.size () < b.size ();
    });
    // for (auto c : v) cerr << c << endl;
    cout << v.back () << '\n';
  }

  return 0;
}