#include <bits/stdc++.h>
using namespace std;

const int N = 3000;
int t , n;
string guai = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
int GS[N] , QD[N] , JB[N];
int gs[N] , qd[N] , jb[N];
int ans[N];
string s;
string sss;
string ssss;
string yl[N] , hl[N];
int ch[N] , seat[N];
void cs(int x,int n) {
    if(x / n != 0) cs(x / n , n);
    sss += guai[x % n];
}

void css(int x,int n) {
    if(x / n != 0) cs(x / n , n);
    ssss += guai[ans[x % n]];
}

bool compare(int s1,int s2) {
    return gs[s1] > gs[s2];
}

bool check() {
    int cnt = 0;
    for(int i = 0;i <= n - 1;i ++) 
        for(int j = 0;j <= n - 1;j ++) {
            int now = i * j;
            ssss = "";
            css(now , n);
            cnt ++;
            hl[cnt] = ssss;
        }
    sort(hl + 1 , hl + 1 + n * n);
    for(int i = 1;i <= n * n;i ++) 
        if(hl[i] != yl[i]) return false;
    return true;
}

bool vis[N];
void dfs(int x,int zfgs) {
    if (x == zfgs + 1) {
        if (check()) {
            for(int i = 0;i <= n - 1;i ++) 
                cout << guai[ans[i]];
            cout << "\n"; 
        }
        return ;
    }
    for (int i = 1; i <= zfgs; i++) {
        if (!vis[i]) {
            vis[i] = true;
            ans[seat[x]] = ch[i];
            dfs(x + 1 , zfgs);
            vis[i] = false;
        }
    }
}

void solve() {
    cin >> n;
    for(int i = 0;i <= 200;i ++) 
        gs[i] = 0 , qd[i] = 0 , GS[i] = QD[i] = 0 , jb[i] = JB[i] = 0;
    for(int i = 0;i <= n - 1;i ++) 
        for(int j = 0;j <= n - 1;j ++) {
            sss = "";
            cs(i * j , n);
            if(sss.size() == 2) GS[sss[0]] ++ , JB[sss[1]] ++;
            if(sss.size() == 1) QD[sss[0]] ++;
        }
    for(int i = 1;i <= n * n;i ++) {
        cin >> s;
        yl[i] = s;
        if(s.size() == 2) gs[s[0]] ++ , jb[s[1]] ++;
        if(s.size() == 1) qd[s[0]] ++;
    }
    sort(yl + 1 , yl + 1 + n * n);
    for(int i = 0;i <= n - 1;i ++) 
        ans[i] = -1;
    int zfgs = 0 , wzgs = 0;
    for(int i = 0;i <= n - 1;i ++) {
        int wocao = 0;
        int num = 0;
        for(int j = 0;j <= n - 1;j ++) {
            if(gs[guai[j]] == GS[guai[i]] && qd[guai[j]] == QD[guai[i]] && jb[guai[j]] == JB[guai[i]]) {
                wocao ++;
                num = j;
            }
        }
        if(wocao == 1) ans[i] = num;
            else seat[++wzgs] = i;
    }
    for(int i = 0;i <= n - 1;i ++) {
        bool flag = 0;
        for(int j = 0;j <= n - 1;j ++) {
            if(ans[j] == -1) continue;
            if(i == ans[j]) {
                flag = 1;
                break;
            }
        }
        if(flag == 0) ch[++zfgs] = i;
    }
    if(wzgs == 0) {
        for(int i = 0;i <= n - 1;i ++) 
            cout << guai[ans[i]];
        cout << "\n"; 
    } else {
        dfs(1 , zfgs);
    }
}

int main() {
    cin >> t;
    while(t --) {
        solve();
    }
    return 0;
}