#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <vector>

using namespace std;

// BEGIN NO SAD
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
typedef vector<int> vi;
#define f first
#define s second
#define derr if(0) cerr
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#define debug(x...) cerr << "\e[91m"<<__func__<<":"<<__LINE__<<" [" << #x << "] = ["; _print(x); cerr << "\e[39m" << flush;
// END NO SAD

template<class Fun>
class y_combinator_result {
  Fun fun_;
public:
  template<class T>
  explicit y_combinator_result(T &&fun): fun_(std::forward<T>(fun)) {}

  template<class ...Args>
  decltype(auto) operator()(Args &&...args) {
    return fun_(std::ref(*this), std::forward<Args>(args)...);
  }
};

template<class Fun>
decltype(auto) y_combinator(Fun &&fun) {
  return y_combinator_result<std::decay_t<Fun>>(std::forward<Fun>(fun));
}

template<class T>
bool updmin(T& a, T b) {
  if(b < a) {
    a = b;
    return true;
  }
  return false;
}
template<class T>
bool updmax(T& a, T b) {
  if(b > a) {
    a = b;
    return true;
  }
  return false;
}
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<vector<ll>> matrix;

const string ALPHA = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
void rsolve() {
  int n;
  cin >> n;
  vector<string> v(n*n);
  for(auto& x: v) cin >> x;
  sort(all(v));
  map<char, int> msdc, lsdc, singlec;
  for(auto out: v) {
    if(sz(out) == 2) msdc[out[0]]++;
    else singlec[out[0]]++;
    lsdc[out.back()]++;
  }
  map<int, int> msdd, lsdd, singled;
  for(int i = 0; i < n; i++) for(int j = 0; j < n; j++) {
    int a = i*j;
    if(a/n) msdd[a/n]++;
    else singled[a%n]++;
    lsdd[a%n]++;
  }
  vector<vector<bool>> poss(n);
  for(int i = 0; i < n; i++) {
    poss[i].resize(n);
    for(int j = 0; j < n; j++) {
      // can i take on value j?
      if(msdc[ALPHA[i]] != msdd[j]) continue;
      if(lsdc[ALPHA[i]] != lsdd[j]) continue;
      if(singlec[ALPHA[i]] != singled[j]) continue;
      poss[i][j] = true;
    }
  }
  vector<bool> assigned(n);
  vector<char> ret(n);
  auto valid = [&]() -> bool {
    vector<string> all;
    for(int i = 0; i < n; i++) for(int j = 0; j < n; j++) {
      int a = i*j;
      string curr = "";
      if(a/n) curr += ret[a/n];
      curr += ret[a%n];
      all.pb(curr);
    }
    sort(all(all));
    return all == v;
  };
  auto dfs = y_combinator([&](auto self, int curr) -> bool {
    if(curr == n) return valid();
    for(int i = 0; i < n; i++) {
      if(!poss[curr][i] || assigned[i]) continue;
      assigned[i] = true;
      ret[i] = ALPHA[curr];
      if(self(curr+1)) return true;
      assigned[i] = false;
    }
    return false;
  });
  assert(dfs(0));
  for(auto out: ret) cout << out; cout << "\n";
}

void solve() {
  int t;
  cin >> t;
  while(t--) rsolve();
}

// what would chika do
// are there edge cases (N=1?)
// are array sizes proper (scaled by proper constant, for example 2* for koosaga tree)
// integer overflow?
// DS reset properly between test cases
// are you doing geometry in floating points
// are you not using modint when you should

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}