#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> pii;
const int N = 55;
inline int ch2id(char c) {
    if (islower(c)) return c - 'a';
    return 26 + c - 'A';
}
inline char id2ch(int x) {
    if (x < 26) return 'a' + x;
    return 'A' + x - 26;
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int Case;
    cin >> Case;
    while (Case--) {
        int n;
        cin >> n;
        vector<pii> cnt(n);
        vector<vector<int>> mp(n, vector<int>(n));
        for (int i = 0; i < n; ++i)
            for (int j = 0; j < n; ++j) {
                int val = i * j;
                cnt[val % n].first += 1;
                if (val >= n) {
                    mp[val / n][val % n] += 1;
                    cnt[val / n].second += 1;
                }
            }
        for (int i = 0; i < n; ++i) sort(mp[i].begin(), mp[i].end());
        vector<pii> cnt_real(n);
        vector<vector<int>> mp_real(n, vector<int>(n));
        for (int i = 0; i < n * n; ++i) {
            string s;
            cin >> s;
            cnt_real[ch2id(s.back())].first += 1;
            if (s.size() == 2) {
                mp_real[ch2id(s[0])][ch2id(s[1])] += 1;
                cnt_real[ch2id(s[0])].second += 1;
            }
        }
        for (int i = 0; i < n; ++i) sort(mp_real[i].begin(), mp_real[i].end());
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < n; ++j) if (cnt[i] == cnt_real[j] && mp[i] == mp_real[j]) {
                cout << id2ch(j);
                break;
            }
        }
        cout << endl;
    }
    return 0;
}