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

typedef long long ll;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	int T;
	cin >> T;
	while(T -- ) {
		int n;
		cin >> n;
		vector<string> s(n * n);
		for(int i = 0; i < n * n; i ++ ) cin >> s[i];

		int cnt = 0;
		vector<array<int, 3>> seq(n);
		map<array<int, 3>, int> mp;
		for(int i = 0; i < n; i ++ ) {
			char ch = (i >= 26 ? (char)(i - 26 + 'A') : (char)(i + 'a'));
			array<int, 3> tmp = {0, 0, 0};
			for(auto str : s) {
				if(str.length() == 1 && str[0] == ch) {
					tmp[0] ++ ;
				} else {
					if(str[0] == ch) tmp[1] ++ ;
					if(str[1] == ch) tmp[2] ++ ;
				}
			}
			seq[i] = tmp;
			if(!mp.count(tmp)) mp[tmp] = cnt ++ ;
		}

		vector<array<int, 3>> arr(n);
		vector<vector<int>> digit(cnt);
		for(int v = 0; v < n; v ++ ) {
			array<int, 3> tmp = {0, 0, 0};
			for(int i = 0; i < n; i ++ ) {
				for(int j = 0; j < n; j ++ ) {
					int val = i * j;
					if(val < n && val == v) {
						tmp[0] ++ ;
					}
					if(val >= n) {
						int v1 = val / n, v2 = val % n;
						if(v1 == v) tmp[1] ++ ;
						if(v2 == v) tmp[2] ++ ;
					}
				}
			}
			arr[v] = tmp;
			digit[mp[tmp]].push_back(v);
		}

		string ans = string(n, '$');
		vector<vector<char>> alpha(cnt);
		for(int i = 0; i < n; i ++ ) {
			if((int)digit[mp[seq[i]]].size() == 1) {
				int pos = digit[mp[seq[i]]][0];
				ans[pos] = (i >= 26 ? (char)(i - 26 + 'A') : (char)(i + 'a'));
			} else {
				char ch = (i >= 26 ? (char)(i - 26 + 'A') : (char)(i + 'a'));
				alpha[mp[seq[i]]].push_back(ch);
			}
		}

		auto get = [&](char ch) -> int {
			if(ch <= 'z') return ch - 'a';
			else return ch - 'A' + 26;
		};

		auto check = [&](string str) -> bool {
			vector<int> p(n);
			for(int i = 0; i < n; i ++ ) p[i] = i;
			sort(p.begin(), p.end(), [&](int a, int b) {
				return str[a] < str[b];
			});

			vector<int> nums(n * n, 0);
			for(int i = 0; i < n; i ++ ) {
				for(int j = 0; j < n; j ++ ) {
					nums[i * j] ++ ;
				}
			}
			for(auto tmps : s) {
				int val = 0;
				for(auto c : tmps) {
					val = val * n + p[get(c)];
				}
				nums[val] -- ;
			}
			for(int i = 0; i < n * n; i ++ ) {
				if(nums[i] != 0) {
					return false;
				}
			}
			return true;
		};

		auto dfs = [&](auto &&dfs, int u) -> bool {
			if(u == cnt) {
				if(check(ans)) {
					cout << ans << '\n';
					return true;
				}
				return false;
			}
			if(alpha[u].empty()) {
				return dfs(dfs, u + 1);
			}

			int len = (int)alpha[u].size();
			vector<int> p(len);
			for(int i = 0; i < len; i ++ ) p[i] = i;
			do {
				for(int i = 0; i < len; i ++ ) {
					ans[digit[u][i]] = alpha[u][p[i]];
				}
				if(dfs(dfs, u + 1)) return true;
			} while(next_permutation(p.begin(), p.end()));

			return false;
		};

		dfs(dfs, 0);
	}
	return 0;
}