#include <map>
#include <set>
#include <array>
#include <cmath>
#include <deque>
#include <bitset>
#include <random>
#include <string>
#include <vector>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
#define all(a) (a).begin(), (a).end()

using namespace std;

map<int, vector<pair<int, int>>> get_context(map<vector<int>, int> cnt) {
  map<int, vector<pair<int, int>>> res;

  for (auto p : cnt) {
    int c = p.second;
    vector<int> v = p.first;

    if (v.size() == 1) {
      res[v[0]].push_back({c, 0});
      continue;
    }

    vector<int> u = v;

    sort(all(u));
    u.resize(unique(all(u)) - u.begin());

    for (auto d : u) {
      if (d == v[0] && d == v[1]) {
        res[d].push_back({c, 1});
      } else if (d == v[0]) {
        res[d].push_back({c, 2});
      } else if (d == v[1]) {
        res[d].push_back({c, 3});
      }
    }
  }
  for (auto& p : res) {
    sort(all(p.second));
  }
  return res;
}

void solve() {
  int n;

  // n = rand() % 52 + 1;

  cin >> n;
  vector<string> s(n * n);
  for (auto& s1 : s) cin >> s1;

  map<vector<int>, int> scnt;
  for (auto s1 : s) {
    vector<int> v1;
    for (auto c : s1) {
      v1.push_back(c - 'a');
    }

    scnt[v1]++;
  }

  vector<int> table;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      table.push_back(i * j);
    }
  }

  map<vector<int>, int> ccnt;
  for (int i : table) {
    if (i < n) {
      ccnt[{i}]++;
    } else {
      ccnt[{i / n, i % n}]++;
    }   
  }

  auto s_context = get_context(scnt);
  auto c_context = get_context(ccnt);

  map<vector<pair<int, int>>, int> c_res;
  for (auto p : c_context) {
    if (c_res.count(p.second)) {
      assert(0);
      cerr << "ERROR\n";
      cerr << "n = " << n << '\n';
      cerr << c_res[p.second] << " <-> " << p.first << '\n';
      return;
    }
    c_res[p.second] = p.first;
  }

  vector<char> res(n);

  for (auto p : s_context) {
    res[c_res[p.second]] = 'a' + p.first;
  }

  for (auto h : res) {
    cout << h;
  }
  cout << '\n';

}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  int t;
  cin >> t;
  while (t--) solve();
  
  return 0;
}