#include<cstdio>

 #include<cstring>

 using namespace std;

 int k, cnt;

 char G[][][], ans[];

 bool dfs(int col){

     if (col == ){

         if (++cnt == k){

             ans[col] = '\0';

             printf("%s\n", ans);

             return true;

         }

         return false;

     }

     bool vis[][];

     memset(vis, false, sizeof(vis));

     for (int i = ; i<; i++)

     for (int j = ; j<; j++)

         vis[i][G[i][j][col] - 'A'] = ;

     for (int i = ; i<; i++)

     if (vis[][i] && vis[][i]){

         ans[col] = i + 'A';

         if (dfs(col + )) return true;

     }

     return false;

 }

 int main()

 {

     /*

     freopen("in.txt", "r", stdin);

     freopen("out.txt", "w", stdout);

     */

     int T;

     scanf("%d", &T);

     while (T--){

         scanf("%d", &k);

         for (int i = ; i<; i++)

         for (int j = ; j<; j++)

             scanf("%s", G[i][j]);

         cnt = ;

         if (!dfs()) puts("NO");

     }

     /*

     fclose(stdin);

     fclose(stdout);

     */

     return ;

 }