// Problem: I. Linguistics Puzzle
// Contest: Codeforces - The 2023 ICPC Asia Hefei Regional Contest (The 2nd Universal Cup. Stage 12: Hefei)
// URL: https://codeforces.com/gym/104857/problem/I
// Memory Limit: 256 MB
// Time Limit: 1000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using VI = vector<int>;
using PII = pair<int,int>;
using VVI = vector<vector<int>>;
const ll mod = 998244353;
const ll INF = 1e18;
const int N = 2e5 + 10;
string s[N];
int a[100][100];
int vis[100];
vector<string> ts;
int tid(char c){
	if(c >= 'a' && c <= 'z'){
		return c - 'a';
	}else{
		return c - 'A' + 26;
	}
}
char toC(int p){
	if(p >= 0 && p < 26){
		return 'a' + p; 
	}else{
		return 'A' + p - 26;
	}
}
string res;
int n;
map<VI , VI> mp;
VVI z(52 , VI(3 , 0));
VVI y(52 , VI(3 , 0));
map<int,char> f;

int dfs(int now){
	if(now == n){
		vector<string> tmp;
		for(int i = 0 ; i < n ; i++){
			for(int j = 0 ; j < n ; j++){
				int p0 = a[i][j] % n;
				int p1 = a[i][j] / n;
				//cout<<res[p0];
				string k;
				if(p1 == 0){
					k += (res[p0]);
				}else{
					k += (res[p1]);
					k += (res[p0]);
				}
				tmp.push_back(k);
				
			}
		}
		sort(tmp.begin() , tmp.end());
		bool flag = 1;
		for(int i = 0 ; i < n * n ; i++){
			if(ts[i] != tmp[i]) flag = 0; 
			
		}
		return flag;
		
	}
	
	auto arr = y[now];
	for(auto c : mp[arr]){
		if(vis[c] == -1){
			//if(now == 1) cout<<c<<" ";
			res.push_back(toC(c));
			vis[c] = now;		
			if(dfs(now + 1)) return 1;
			vis[c] = -1;
			res.pop_back();
		}
	
	}
	return 0;

}

void solve(){
	res.clear();
	mp.clear();
	ts.clear();
	cin>>n;
	z.assign(n , VI(3 , 0));
	y.assign(n , VI(3 , 0));
	for(int i = 0 ; i < n * n ; i++){
		cin>>s[i];
		if(s[i].size() == 1){
			z[tid(s[i][0])][0]++;
		}else{
			z[tid(s[i][1])][0]++;
			z[tid(s[i][0])][1]++;
			if(s[i][0] == s[i][1]){
				z[tid(s[i][0])][2]++;
			}
			
		}
		ts.push_back(s[i]);
	}
	sort(ts.begin() , ts.end());
	//cout<<tid('d');
	for(int i = 0 ; i < n ; i++){
		vis[i] = -1;
		mp[z[i]].push_back(i);
	}
	
	for(int i = 0 ; i < n ; i++){
		for(int j = 0 ; j < n ; j++){
			int w = i * j;
			y[w % n][0]++;
			if(w >= n) {
				y[w /n][1]++;
				if(w / n == w % n) y[w / n][2]++;
			}
				
		}
	}
	
	dfs(0);
	cout<<res;
	cout<<endl;
}

int main() {
	ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    for(int i = 0 ; i <= 52 ; i++){
    	for(int j = 0 ; j <= 52 ; j++){
    		a[i][j] = i * j;
    	
    	}
    }
    
    int T;cin>>T;while(T--)solve();
    
    
    
    // 0  0   0      0 
    // 0  1   2      3
    // 0  2   4(10)  6(12) 
    // 0  3   6(12)  9(22);
    
}


//5 4 3 2 1
//5 4 3 2 1