#include <bits/stdc++.h>
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define st first
#define nd second
using namespace std;
typedef long long ll;
const int mod = 1e9 + 7;
const int N = 1e6 + 5;

int f(char c) {
	if ('a' <= c && c <= 'z')
		return c - 'a';
	return c - 'A' + 26;
}

char g(int x) {
	if (x < 26)
		return (char)(x + 'a');
	return (char)(x - 26 + 'A');
}

void solve() {
	int n; cin >> n;
	vector<map<string, int>> s(2);
	for (int i = 0; i < n * n; i++) {
		string t; cin >> t;
		// int p = (i / n) * (i % n);
		// int x = p / n, y = p % n;
		// string t;
		// if (x != 0)
		// 	t.push_back(g(x));
		// t.push_back(g(y));
		s[sz(t) - 1][t]++;
	}
	string ans(n, '#');
	vector<int> vis(2 * n);
	while (1) {
		bool change = false;
		vector<int> cnt(n);
		map<int, int> tmp;
		for (int i = 0; i < n; i++) {
			for (int j = 0; i * j < n && j < n; j++)
				if (ans[i * j] == '#')
					cnt[i * j]++;
		}
		for (int i = 0; i < n; i++)
			if (cnt[i] != 0)
				tmp[cnt[i]]++;
		for (int i = 0; i < n; i++) {
			if (cnt[i] == 0 || tmp[cnt[i]] > 1)
				continue;
			for (auto [ss, j]: s[0]) {
				if (j == cnt[i] && vis[f(ss[0])] == 0) {
					change = true;
					ans[i] = ss[0];
					vis[f(ss[0])] = i + 1;
					break;
				}
			}
		}
		// cout << "? " << ans << endl;
		vector<map<int, string>> arr(1 + 2 * n);
		for (auto [ss, cc]: s[1]) {
			int msk = (vis[f(ss[0])] > 0) | (2 * (vis[f(ss[1])] > 0));
			if (msk == 3)
				continue;
			int idx = 0;
			if (msk != 0)
				idx = 1 + vis[f(ss[msk - 1])] - 1 + n * (msk == 2);
			arr[idx][cc] = ss;
			// cout << idx << " " << ss << " " << cc << " " << msk << endl;
		}
		vector<map<int, int>> nxt(1 + 2 * n);
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				if (i * j < n)
					continue;
				int x = (i * j) / n, y = (i * j) % n;
				int msk = (ans[x] != '#') | (2 * (ans[y] != '#'));
				if (msk == 3)
					continue;
				int idx = 0;
				if (msk != 0)
					idx = 1 + (msk == 1 ? x : n + y);
				// cout << idx << " " << i * j << " " << x << y << " " << msk << endl;
				nxt[idx][i * j]++;
			}
		}
		for (int i = 0; i <= 2 * n; i++) {
			map<int, int> rev;
			for (auto [p, cc]: nxt[i])
				rev[cc]++;
			for (auto [p, cc]: nxt[i]) {
				if (rev[cc] > 1)
					continue;
				// cout << i << " " << p << " " << cc << endl;
				string ss = arr[i][cc];
				int x = p / n, y = p % n;
				ans[x] = ss[0];
				ans[y] = ss[1];
				vis[f(ss[0])] = x + 1;
				vis[f(ss[1])] = y + 1;
				change = true;
			}
		}
		// cout << " ! " << ans << endl;
		if (change == false)
			break;
	}
	cout << ans << "\n";
}

int main() {
	ios_base::sync_with_stdio(false); cin.tie(0);
	int t; cin >> t; while (t--) solve();
}