#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <vector>
#include <set>
#include <queue>
#include <map>
#include <unordered_map>
#include <iomanip>
#define endl '\n'
#define int long long
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define rep2(i,a,b) for(int i=(a);i>=(b);i--)
using namespace std;

template<typename T>
void cc(const vector<T> &tem) {
    for (const auto &x: tem) cout << x << ' ';
    cout << endl;
}

template<typename T>
void cc(const T &a) { cout << a << endl; }

template<typename T1, typename T2>
void cc(const T1 &a, const T2 &b) { cout << a << ' ' << b << endl; }

template<typename T1, typename T2, typename T3>
void cc(const T1 &a, const T2 &b, const T3 &c) { cout << a << ' ' << b << ' ' << c << endl; }

void cc(const string &s) { cout << s << endl; }

void fileRead() {
#ifdef LOCALL
    freopen("D:\\AADVISE\\Clioncode\\untitled2\\in.txt", "r", stdin);
    freopen("D:\\AADVISE\\Clioncode\\untitled2\\out.txt", "w", stdout);
#endif
}

void kuaidu() { ios::sync_with_stdio(false), cin.tie(0), cout.tie(0); }

inline int max(int a, int b) {
    if (a < b) return b;
    return a;
}

inline double max(double a, double b) {
    if (a < b) return b;
    return a;
}

inline int min(int a, int b) {
    if (a < b) return a;
    return b;
}

inline double min(double a, double b) {
    if (a < b) return a;
    return b;
}

void cmax(int &a, const int &b) { if (b > a) a = b; }
void cmin(int &a, const int &b) { if (b < a) a = b; }
void cmin(double &a, const double &b) { if (b < a) a = b; }
void cmax(double &a, const double &b) { if (b > a) a = b; }
using PII = pair<int, int>;
using i128 = __int128;
using vec_int = std::vector<int>;
using vec_char = std::vector<char>;
using vec_double = std::vector<double>;
using vec_int2 = std::vector<std::vector<int> >;
using que_int = std::queue<int>;


//--------------------------------------------------------------------------------
const int N = 1e5 + 10;
const int M = 1e6 + 10;
const int mod = 1e9 + 7;
const int INF = 1e16;
int n, m, T;
map<PII, vec_int> mp, mp2;
vec_int res;
//数字x在十位和个位上出现的次数是多少
PII biao[N];

// 字符i选择的数字是谁
int book[N];

//数字i有没有被选择
bool xuan[N];

//字符i的出现的次数
PII to[N];

vector<string> qs;
//--------------------------------------------------------------------------------
//struct or namespace:

//--------------------------------------------------------------------------------

void pre() {
    rep(i, 0, n-1)
        rep(j, 0, n-1) {
            int l = i * j;
            res.push_back(l);
            if (l < n) {
                biao[l].second++;
            }
            else {
                biao[l / n].first++;
                biao[l % n].second++;
            }
        }
    sort(res.begin(), res.end());
    rep(i, 0, n-1) {
        mp[biao[i]].push_back(i);
    }
}

int fan(char x) {
    if (x <= 'z' and x >= 'a') return x - 'a';
    return 26 + x - 'A';
}

char back(int x) {
    if (x <= 25) return char(x + 'a');
    return char(x - 26 + 'A');
}

bool check() {
    vec_int tem;
    for (auto &s: qs) {
        if (s.size() == 1) tem.push_back(book[fan(s[0])]);
        else {
            tem.push_back(book[fan(s[0])] * n + book[fan(s[1])]);
        }
    }
    sort(tem.begin(), tem.end());
    rep(i, 0, n*n-1) {
        if (tem[i] != res[i]) return 0;
    }

    rep(i, 1, n) {

    }
    return 1;
}

int dfs(int x) {
    if (x == n) {
        if (check()) return 1;
        return 0;
    }
    for (auto &l: mp[to[x]]) {
        if (xuan[l]) continue;
        xuan[l] = 1;
        book[x] = l;
        if (dfs(x + 1)) return 1;
        xuan[l] = 0;
    }
    return 0;
}

signed main() {
    fileRead();

    kuaidu();
    T = 1;
    cin >> T;
    while (T--) {
        cin >> n;

        mp.clear();
        mp2.clear();
        res.clear();

        rep(i, 0, n) {
            to[i] = {0, 0};
            xuan[i] = 0;
            biao[i] = {0, 0};
        }
        qs.clear();

        pre();
        rep(i, 1, n*n) {
            string s;
            cin >> s;
            qs.push_back(s);
            if (s.size() == 1) {
                to[fan(s[0])].second++;
            }
            else {
                to[fan(s[0])].first++;
                to[fan(s[1])].second++;
            }
        }
        dfs(0);
        rep(i, 0, n-1) {
            rep(j, 0, n-1) {
                if (book[j] == i) {
                    cout << back(j);
                    break;
                }
            }
        }
        cout << endl;
    }
    return 0;
}

/*


*/