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

#define all(x) begin(x), end(x)
#ifdef CKISEKI
#include <experimental/iterator>
#define debug(a...) debug_(#a, a)
#define orange(a...) orange_(#a, a)
void debug_(auto s, auto ...a) {
  cerr << "\e[1;32m(" << s << ") = (";
  int f = 0;
  (..., (cerr << (f++ ? ", " : "") << a));
  cerr << ")\e[0m\n";
}
void orange_(auto s, auto L, auto R) {
  cerr << "\e[1;33m[ " << s << " ] = [ ";
  using namespace experimental;
  copy(L, R, make_ostream_joiner(cerr, ", "));
  cerr << " ]\e[0m\n";
}
#else
#define safe ((void)0)
#define debug(...) safe
#define orange(...) safe
#endif

int enc(string s) {
  int r = 0;
  for (auto c : s)
    r = r * 128 + c;
  return r;
}

string dec(int x) {
  string s(1, x % 128);
  if (x > 128)
    s = char(x / 128) + s;
  return s;
}

bool used[128];
int hc[128];
int cnt[128 * 128];

vector<int> same[1000];

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int t;
  cin >> t;
  while (t--) {
    memset(used, 0, sizeof(used));
    memset(hc, 0, sizeof(hc));
    memset(cnt, 0, sizeof(cnt));
    for (int i = 0; i < 1000; ++i)
      same[i].clear();

    int n;
    cin >> n;
    vector<string> a(n * n);
    for (auto &ai : a)
      cin >> ai;

    string ans(n, '?');

    for (auto ai : a)
      cnt[enc(ai)]++;

    for (int i = 0; i < 128; ++i) {
      if (cnt[i] == 2 * n - 1)
        ans[0] = dec(i)[0];
    }

    for (auto &ai : a)
      if (ai.size() == 1)
        ai = ans[0] + ai;

    vector<int> tot(n);
    for (int i = 0; i < n; ++i)
      for (int j = 0; j < n; ++j)
        tot[(i * j) / n]++;
    for (int i = 0; i < n; ++i) {
      same[tot[i]].push_back(i);
    }

    for (auto ai : a)
      hc[int(ai[0])]++;

    auto dfs = [&](auto self, int i) -> bool {
      if (i == 128)
        return true;
      if (hc[i] == 0)
        return self(self, i + 1);
      for (int x : same[hc[i]]) {
        // x is i
        if (used[x])
          continue;
        used[x] = true;
        ans[x] = i;
        if (self(self, i + 1))
          return true;
        ans[x] = '?';
        used[x] = false;
      }
      return false;
    };
    assert (dfs(dfs, 0));

    set<char> s;
    for (int i = 0; i < n; ++i) {
      if (i < 26)
        s.insert(i + 'a');
      else
        s.insert(i - 26 + 'A');
    }
    for (auto x : ans) {
      s.erase(x);
    }
    ans.back() = *s.begin();

    cout << ans << '\n';
  }
  return 0;
}