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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

void jikken(int n, bool show){
	vector<map<int,int>> mp(n);
	rep(i,0,n){
		rep(j,0,n){
			mp[i * j / n][i * j % n]++;
		}
	}

	set<vector<int>> st;
	rep(i,1,n){
		vector<int> a;
		for (auto [x, c]: mp[i]){
			a.push_back(c);
		}
		sort(a.begin(), a.end());
		st.insert(a);
	}
	cout << n << ' ' << (int)st.size() << '\n';
	if (n-1 != (int)st.size()) cout << "WARNING " << n << endl;

	if (show){
		rep(i,0,n){
			cout << n << " ABOUT " << i << endl;
			for (auto [x, c]: mp[i]){
				cout << x << " * " << c << endl;
			}
			cout << endl;
		}
	}

}

string solve(int n, vector<string> s){
	string shudai = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
	assert(shudai.size() == 52);

	map<char,int> fuku;
	rep(i,0,52){
		fuku[shudai[i]] = i;
	}

	map<char,int> len_1;
	rep(i,0,n * n){
		if ((int)s[i].size() == 1){
			len_1[s[i][0]]++;
		}
	}

	map<char,int> taio;
	vector<char> modoso(n, '$');

	for (auto [i,x]: len_1){
		if (x == 2 * n - 1){
			taio[i] = 0;
			modoso[0] = i;
		}
		if (x == 1){
			taio[i] = 1;
			modoso[1] = i;
		}
	}

	
	vector<map<int,int>> mp_orig(n);
	rep(i,0,n){
		rep(j,0,n){
			mp_orig[i * j / n][i * j % n]++;
		}
	}

	vector<map<char,int>> mp_chiper(n);
	rep(i,0,n*n){
		if ((int)s[i].size() == 2){
			mp_chiper[fuku[s[i][0]]][s[i][1]]++;
		}
	}

	map<vector<int>,int> gogo;
	set<vector<int>> kinshi;
	rep(i,1,n){
		vector<int> tmp;
		for(auto [j,d]: mp_orig[i]){
			tmp.push_back(d);
		}
		sort(tmp.begin(), tmp.end());
		if (gogo.find(tmp) != gogo.end()){
			kinshi.insert(tmp);
		}else{
			gogo[tmp] = i;
		}
	}


	vector<char> blacklist;
	rep(i,0,n){
		char x = shudai[i];
		if (taio.find(x) != taio.end()) continue;

		vector<int> tmp;
		for (auto [j,d]: mp_chiper[fuku[x]]){
			tmp.push_back(d);
		}

		sort(tmp.begin(), tmp.end());
		if (kinshi.find(tmp) != kinshi.end()){
			blacklist.push_back(x);
		}else{
			taio[x] = gogo[tmp];
			modoso[gogo[tmp]] = x;
		}
	}

	if ((int)blacklist.size() > 0){
		vector<int> no_place;
		rep(i,0,n){
			if (modoso[i] == '$') no_place.push_back(i);
		}
		vector<int> moto;
		rep(i,0,n){
			rep(j,0,n){
				moto.push_back(i * j);
			}
		}

		sort(moto.begin(), moto.end());
		int k = blacklist.size();
		vector<int> p(k);
		iota(p.begin(), p.end(), 0);
		do{
			rep(i,0,k){
				taio[blacklist[i]] = no_place[p[i]];
				modoso[no_place[p[i]]] = blacklist[i];
			}
			vector<int> tar;
			rep(i,0,n*n){
				if (s[i].size() == 1){
					tar.push_back(taio[s[i][0]]);
				}else{
					tar.push_back(taio[s[i][0]] * n + taio[s[i][1]]);
				}
			}
			sort(tar.begin(), tar.end());
			if (tar == moto){
				break;
			}
			rep(i,0,k){
				taio.erase(blacklist[i]);
				modoso[no_place[p[i]]] = '$';
			}
		}while(next_permutation(p.begin(), p.end()));
	}

	string ans;
	rep(i,0,n){
		ans += modoso[i];
	}

	return ans;

}


void assertion(){
	random_device seed_gen;
	mt19937 engine(seed_gen());
	uniform_int_distribution<int> dist(2, 52);
	string shudai = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

	rep(num,0,10000){
		int n = dist(engine);
		string s2 = shudai.substr(0, n);
		shuffle(s2.begin(), s2.end(), engine);
		vector<string> s;
		rep(i,0,n){
			rep(j,0,n){
				if (i*j/n == 0){
					string x = "";
					x += s2[i*j%n];
					s.push_back(x);
				}else{
					string x = "";
					x += s2[i*j/n];
					x += s2[i*j%n];
					s.push_back(x);
				}
			}
		}
		shuffle(s.begin(), s.end(), engine);
		string ans = solve(n, s);
		if (ans != s2){
			cout << "ASSERTION FAILED. " << num << endl;
			cout << n << endl;
		}
	}
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	int t; cin >> t;
	while(t--){
		int n; cin >> n;
		vector<string> s(n * n);
		rep(i,0,n * n){
			cin >> s[i];
		}
		cout << solve(n, s) << '\n';
	}
}

answer.code:5:9: fatal error: atcoder/all: No such file or directory
    5 | #include<atcoder/all>
      |         ^~~~~~~~~~~~~
compilation terminated.