#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


int tr(char c) {
  if (islower(c)) return 0 + (c - 'a');
  if (isupper(c)) return 26 + (c - 'A');
  assert(false);
}
char rt(int x) {
  if (0 <= x && x < 26) return 'a' + (x - 0);
  if (26 <= x && x < 52) return 'A' + (x - 26);
  assert(false);
}

constexpr int MAX_N = 52;
map<vector<int>, vector<int>> cache[MAX_N + 1];

int main() {
  for (int N = 1; N <= MAX_N; ++N) {
    vector<pair<vector<int>, int>> infos(N);
    for (int u = 0; u < N; ++u) {
      infos[u] = make_pair(vector<int>{0, 0, 0}, u);
    }
    for (int a = 0; a < N; ++a) for (int b = 0; b < N; ++b) {
      const int c = a * b;
      if (c < N) {
        ++infos[c].first[0];
      } else {
        ++infos[c / N].first[1];
        ++infos[c % N].first[2];
      }
    }
    // cerr << N << ": " << infos << endl;
    for (int u = 0; u < N; ++u) {
      cache[N][infos[u].first].push_back(u);
    }
#ifdef LOCAL
    sort(infos.begin(), infos.end());
    for (int i = 0, j = 0; i < N; i = j) {
      for (; j < N && infos[i].first == infos[j].first; ++j) {}
      if (j - i >= 2) {
        cerr << N << ": "; pv(infos.begin() + i, infos.begin() + j);
      }
    }
#endif
  }
  
  char buf[110];
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    int N;
    scanf("%d", &N);
    vector<string> S(N * N);
    for (int i = 0; i < N * N; ++i) {
      scanf("%s", buf);
      S[i] = buf;
    }
    sort(S.begin(), S.end());
    
    vector<vector<int>> keys(N);
    for (int x = 0; x < N; ++x) {
      keys[x] = {0, 0, 0};
    }
    for (int i = 0; i < N * N; ++i) {
      if (S[i].size() == 1) {
        ++keys[tr(S[i][0])][0];
      } else {
        ++keys[tr(S[i][0])][1];
        ++keys[tr(S[i][1])][2];
      }
    }
    vector<vector<int>> uss(N);
    for (int x = 0; x < N; ++x) {
      uss[x] = cache[N][keys[x]];
    }
// cerr<<"uss = "<<uss<<endl;
    vector<int> xs;
    for (int x = 0; x < N; ++x) if (uss[x].size() == 2) {
      xs.push_back(x);
    }
    const int xsLen = xs.size();
    for (int p = 0; p < 1 << xsLen; ++p) {
      string ans(N, '?');
      for (int x = 0; x < N; ++x) if (uss[x].size() == 1) {
        ans[uss[x][0]] = rt(x);
      }
      for (int j = 0; j < xsLen; ++j) {
        ans[uss[xs[j]][p >> j & 1]] = rt(xs[j]);
      }
      vector<string> ss(N * N);
      for (int a = 0; a < N; ++a) for (int b = 0; b < N; ++b) {
        const int i = a * N + b;
        const int c = a * b;
        if (c < N) {
          ss[i] = string() + ans[c];
        } else {
          ss[i] = string() + ans[c / N] + ans[c % N];
        }
      }
      sort(ss.begin(), ss.end());
      if (S == ss) {
        puts(ans.c_str());
        goto done;
      }
    }
    assert(false);
   done:{}
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}