#include <bits/stdc++.h>
using namespace std;
#define int long long
#define N 300005
#define endl '\n'
#define PII pair<int, int>
int n;
string s;
map<char, int> vis[2], cnt[2];
map<PII, queue<char>> zm;
map<int, queue<char>> mz;
map<char, char> ot;
map<char, bool> chk;
char getchr(int x)
{
    if (x < 26)
        return x + 'a';
    return x - 26 + 'A';
}
void solve()
{
    for (int i = 0; i <= 1; i++)
    {
        vis[i].clear();
        cnt[i].clear();
    }
    ot.clear();
    zm.clear();
    mz.clear();
    chk.clear();
    cin >> n;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
        {
            cin >> s;
            int tmp = s.size();
            tmp--;
            for (int j = 0; s[j]; j++)
                vis[tmp - j][s[j]]++;
        }
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
        {
            int gw = (i * j) % n, sw = (i * j) / n;
            cnt[0][getchr(gw)]++;
            if (sw)
                cnt[1][getchr(sw)]++;
        }
    /*for (auto [c, js] : cnt[0])
        cout << c << ' ' << js << endl;
    for (auto [c, js] : cnt[1])
        cout << c << ' ' << js << endl;
    cout << "===================\n";
    for (auto [c, js] : vis[0])
        cout << c << ' ' << js << endl;
    for (auto [c, js] : vis[1])
        cout << c << ' ' << js << endl;*/
    for (auto [c, js] : cnt[0])
        if (!chk[c] && js)
        {
            zm[{js, cnt[1][c]}].push(c);
            chk[c] = 1;
        }
    for (auto [c, js] : cnt[1])
        if (!chk[c] && js)
        {
            mz[js].push(c);
            chk[c] = 1;
        }
    chk.clear();
    for (auto [c, js] : vis[0])
        if (!chk[c] && js)
        {
            ot[zm[{js, vis[1][c]}].front()] = c;
            zm[{js, vis[1][c]}].pop();
            chk[c] = 1;
        }
    for (auto [c, js] : vis[1])
        if (!chk[c] && js)
        {
            ot[mz[js].front()] = c;
            mz[js].pop();
            chk[c] = 1;
        }
    for (auto [c, s] : ot)
        cout << s;
    cout << endl;
}
signed main()
{
    int t = 1;
    cin >> t;
    while (t--)
        solve();
    return 0;
}