#include <bits/stdc++.h>
#define x first
#define y second
using namespace std;
using ll=long long;
using vi=vector<int>;
using vl=vector<ll>;
using pii=pair<int,int>;
#define pb push_back
#define all(a) begin(a),end(a)

const int N=300010,MOD=1e9+7;
const char en='\n';
const ll LLINF=1ll<<60;

int t,n;
map<pair<char,char>,int> occ;
map<pii,int> oci;

using eqc=vector<pair<set<char>,set<int>>>;
eqc neweq(eqc ol)
{
	eqc re;
	for (auto x: ol)
	{
		map<pair<vector<multiset<int>>,vector<multiset<int>>>,set<char>> chm;
		map<pair<vector<multiset<int>>,vector<multiset<int>>>,set<int>> inm;
		for (auto y: x.x)
		{
			//char
			vector<multiset<int>> prvi,drugi;
			for (auto ec: ol)
			{
				multiset<int> occp,occd;
				for (auto ch: ec.x)
				{
					occp.insert(occ[{y,ch}]);
					occd.insert(occ[{ch,y}]);
				}
				prvi.pb(occp);
				drugi.pb(occd);
			}
			/*cout<<"C"<<y<<en;
			cout<<"prvi:"<<en;
			for (auto u: prvi)
			{
				for (auto v: u) cout<<v<<' ';
				cout<<endl;
			}
			cout<<"drugi:"<<en;
			for (auto u: drugi)
			{
				for (auto v: u) cout<<v<<' ';
				cout<<endl;
			}*/
			chm[{prvi,drugi}].insert(y);
		}
		for (auto y: x.y)
		{
			//int
			vector<multiset<int>> prvi,drugi;
			for (auto ec: ol)
			{
				multiset<int> occp,occd;
				for (auto ch: ec.y)
				{
					occp.insert(oci[{y,ch}]);
					occd.insert(oci[{ch,y}]);
				}
				prvi.pb(occp);
				drugi.pb(occd);
			}
			/*cout<<"I"<<y<<en;
			cout<<"prvi:"<<en;
			for (auto u: prvi)
			{
				for (auto v: u) cout<<v<<' ';
				cout<<endl;
			}
			cout<<"drugi:"<<en;
			for (auto u: drugi)
			{
				for (auto v: u) cout<<v<<' ';
				cout<<endl;
			}*/
			inm[{prvi,drugi}].insert(y);
		}
		for (auto x: chm)
		{
			assert(chm[x.x].size()==inm[x.x].size());
			re.pb({chm[x.x],inm[x.x]});
		}
	}
	return re;
}

set<char> sup;
set<int> sm1;

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	sup.insert('?');
	sm1.insert(-1);
	cin>>t;
	while (t--)
	{
		cin>>n;
		occ.clear();
		oci.clear();
		for (int i=0;i<n;++i) for (int j=0;j<n;++j)
		{
			int x=(i*j)/n,y=(i*j)%n;
			if (x==0) x=-1;
			//cout<<x<<' '<<y<<en;
			++oci[{x,y}];
		}
		for (int i=0;i<n*n;++i)
		{
			string s;
			cin>>s;
			if (s.size()==1) s="?"+s;
			//cout<<s[0]<<' '<<s[1]<<endl;
			++occ[{s[0],s[1]}];
		}
		eqc cu;
		cu.pb({sup,sm1});
		set<char> sc;
		set<int> si;
		for (int i=0;i<n;++i) si.insert(i);
		for (int i=0;i<n;++i)
		{
			if (i<26) sc.insert('a'+i);
			else sc.insert('A'+i-26);
		}
		cu.pb({sc,si});
		while (1)
		{
			eqc nov=neweq(cu);
			if (nov==cu) break;
			cu=nov;
		}
		assert((int)cu.size()==n+1);
		for (int i=0;i<n;++i)
		{
			for (auto x: cu)
			{
				char ch=*x.x.begin();
				int in=*x.y.begin();
				if (in==i) cout<<ch;
			}
		}
		cout<<endl;
	}
}