/*
Undo the destiny.
*/
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define FO(x) {freopen(#x".in","r",stdin);freopen(#x".out","w",stdout);}
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mp make_pair
const int N = 55;
const int M = 55 * 55;
int n;
int cnt1[N],cnt2[N][N],num[M],ans[N];
bool vis[N];
int getnum(char now) {
    if (now <= 'z' && now >= 'a') return now - 'a';
    else return now - 'A' + 26;
}
char getans(int now) {
    if (now < 26) return now + 'a';
    else return now - 26 + 'A';
}
bool dfs(int dep) {
    if (dep >= n) {
        return 1;
    }
    //dep是数字，现在枚举一个出现次数相同的字符找
    //i 是字符
    bool hav = 0;
    for (int i = 0;i < n;++i) {
        if (vis[i]) continue;
        if (cnt1[i] == num[dep]) {
            bool flag = 1;
            for (int j = 0;j < dep;++j) {
                if ((cnt2[ans[j]][i] != num[j * n + dep] && j) && (cnt2[i][ans[j]] != num[dep * n + j] && dep)) {
                    //cout << "errL"<<dep << " " << i << " " << ans[j] << "\n" << cnt2[ans[j]][i] << " " << num[ans[j]*n+i]<<" " << cnt2[i][ans[j]] << " " << num[i * n + ans[j]]<< "\n";
                    flag = 0;
                }
            }
            //cout << dep << " " << i << " " << flag << "\n";
            if (flag) {
                ans[dep] = i;
                vis[i] = 1;
                if (!dfs(dep+1)) vis[i] = 0,ans[dep] = 0;
                else hav = 1;
            }
        }
    }
    return hav;
}
void solve() {
    cin >> n;
    memset(num,0,sizeof num);
    memset(cnt1,0,sizeof cnt1);
    memset(cnt2,0,sizeof cnt2);
    memset(ans,0,sizeof ans);
    memset(vis,0,sizeof vis);
    for (int i = 0;i < n;++i) {
        for (int j = 0;j < n;++j) {
            num[i*j]++;
        }
    }
    //cout << "num:";
    //for (int i = 0;i < n*n;++i) cout << num[i] << " ";
    //cout <<"\n";
    for (int i = 0;i < n;++i) {
        for (int j = 0;j < n;++j) {
            string now;cin >> now;
            if (now.size() == 1) cnt1[getnum(now[0])]++;
            else cnt2[getnum(now[0])][getnum(now[1])]++;
        }
    }
    /*for (int i = 0;i < n;++i) {
        for (int j = 0;j < n;++j) {
            cout << cnt2[i][j] << " ";
        }
        cout << "\n";
    }
    cout << "cnt1:";
    for (int i = 0;i < n;++i) cout << cnt1[i] << " ";
    cout << "\n";*/
    dfs(0);
    for (int i = 0;i < n;++i) cout << getans(ans[i]);
    cout << "\n";
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);//cout.tie(0);
    int T;cin >> T;
    while (T--) solve();
    return 0;
}
/*
1
4
d d d d d c b a d b cd cb d a cb bc
*/