#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];

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

		vector<int> c1(n, 0), c1_(n, 0), s1(n, 0), s1_(n, 0), s2(n, 0), s2_(n, 0);
		vector<vector<int>> c(n, vector<int> (n, 0)), c_(n, vector<int> (n, 0));
		for(int i = 0; i < n * n; i ++ ) {
			if(s[i].length() == 1) {
				c1[getid(s[i][0])] ++ ;
			} else {
				s1[getid(s[i][0])] ++ , s2[getid(s[i][1])] ++ ;
				c[getid(s[i][0])][getid(s[i][1])] ++ ;
			}
		}
		for(int i = 0; i < n; i ++ ) {
			for(int j = 0; j < n; j ++ ) {
				int v = i * j;
				if(v < n) {
					c1_[v] ++ ;
				} else {
					s1_[v / n] ++ , s2_[v % n] ++ ;
					c_[v / n][v % n] ++ ;
				}
			}
		}

		vector<int> alpha;
		vector<bool> has(n, false);
		for(int i = 0; i < n; i ++ ) {
			for(int j = i + 1; j < n; j ++ ) {
				if(c1[i] == c1[j] && s1[i] == s1[j] && s2[i] == s2[j]) {
					has[i] = has[j] = true;
				}
			}
		}
		for(int i = 0; i < n; i ++ ) {
			if(has[i]) {
				alpha.push_back(i);
			}
		}

		vector<int> ans(n, -1);
		vector<bool> used(n, false);
		for(int i = 0; i < n; i ++ ) {
			for(int j = 0; j < n; j ++ ) {
				if(!has[i] && c1[i] == c1_[j] && s1[i] == s1_[j] && s2[i] == s2_[j]) {
					ans[i] = j, used[j] = true;
				}
			}
		}

		vector<int> num;
		for(int i = 0; i < n; i ++ ) {
			if(!used[i]) {
				num.push_back(i);
			}
		}

		int len = (int)num.size();
		vector<int> p(len);
		for(int i = 0; i < len; i ++ ) p[i] = i;
		do {
			for(int i = 0; i < len; i ++ ) {
				ans[num[p[i]]] = alpha[i];
			}

			bool ok = true;
			for(int i = 0; i < n; i ++ ) {
				if(c1[i] != c1_[ans[i]] || s1[i] != s1_[ans[i]] || s2[i] != s2_[ans[i]]) {
					ok = false;
				}
			}
			for(int i = 0; i < n; i ++ ) {
				for(int j = 0; j < n; j ++ ) {
					if(c[i][j] != c_[ans[i]][ans[j]]) {
						ok = false;
					}
				}
			}
			if(ok) {
				vector<int> res(n);
				for(int i = 0; i < n; i ++ ) res[i] = i;
				sort(res.begin(), res.end(), [&](int a, int b) {
					return ans[a] < ans[b];
				});
				for(int i = 0; i < n; i ++ ) {
					cout << (res[i] < 26 ? (char)(res[i] + 'a') : (char)(res[i] - 26 + 'A'));
				}
				cout << '\n';
				break;
			}
		} while(next_permutation(p.begin(), p.end()));
	}
	return 0;
}