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

#define rep(i,a,b) for (int i=a; i<b; i++)
#define trav(a,b) for (auto a : b)
#define all(x) begin(x),end(x)
#define lld long long

using i64 = long long;


vector<pair<int,int> > calc(int n){
	vector<pair<int,int> > cnt(n,{0,0});
	rep(i,0,n){
		rep(j,0,n){
			cnt[(i*j)/n].first++;
			cnt[(i*j)%n].second++;
		}
	}
	cnt[0].first=0;
	int tot=0;
	rep(i,0,n){
		bool W=true;
		rep(j,0,n){
			if(cnt[i]==cnt[j] && i!=j)W=false;
		}
		tot+=W;
		//cout<<cnt[i].first<<" "<<cnt[i].second<<endl;
	}
	//cout<<n<<" "<<n-tot<<endl;
	return cnt;
}

string alphabet="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
int idx[256];
void solve() {
	//rep(i,1,52){
		//cout<<i<<endl;
		//check(i);
	//}
	int n;
	cin>>n;
	vector<string> V;
	vector<pair<int,int> > Tot=calc(n);
	vector<pair<int,int>> cnt(n,{0,0});
	rep(i,0,n*n){
		string s;
		cin>>s;
		V.push_back(s);
		if(s.size()>1){
			cnt[idx[(int)s[0]]].first++;
			cnt[idx[(int)s[1]]].second++;
		}else{
			cnt[idx[(int)s[0]]].second++;
		}
	}
	sort(V.begin(),V.end());
	//trav(a,cnt)cout<<a.first<<" "<<a.second<<endl;
	//trav(a,Tot)cout<<a.first<<" "<<a.second<<endl;
	vector<int> per(n,-1);
	vector<int> inv(n,-1);
	
	rep(i,0,n){
		int ans=-2;
		rep(j,0,n){
			if(Tot[j]==cnt[i]){
				if(ans==-2){
					ans=j;
				}else{
					ans=-1;
				}
			}
		}
		per[i]=ans;
		if(ans!=-1)inv[ans]=i;
	}
	bool DET=true;
	vector<pair<int,int> >CNT(n);
	vector<pair<int,int> >EXP(n);
	do{
		
		DET=false;
		rep(i,0,n){
			if(per[i]==-1){
				DET=true;
				int ans=-2;
				rep(j,0,n){
					if(inv[j]==-1){
						rep(k,0,n)CNT[k]={0,0},EXP[k]={0,0};
						
						trav(a,V){
							if(a.size()==2){
								if(a[0]==alphabet[i]){
									assert(idx[a[1]]!=-1);
									if(per[idx[(int)a[1]]]!=-1){
										CNT[per[(int)idx[a[1]]]].first++;
									}
								}
								if(a[1]==alphabet[i]){
									assert(idx[a[0]]!=-1);
									if(per[(int)idx[a[0]]]!=-1){
										CNT[per[(int)idx[a[0]]]].second++;
									}
								}
							}
						}
						
						rep(k,0,n){
							rep(l,0,n){
								int u=k*l;
								if(u>=n){
									int B2=u/n;
									int B1=u%n;
									if(B2==j){
										EXP[B1].first++;
									}
									if(B1==j){
										EXP[B2].second++;
									}
								}
							}
						}
						
						//cout<<i<<endl;
						bool W=true;
						rep(k,0,n){
							if(inv[k]!=-1){
								if(EXP[k]!=CNT[k]){
									W=false;
								}
							}
						}
						if(W){
							if(ans==-2){
								ans=j;
							}else{
								ans=-1;
							}
						}
					}
				}
				
				//cout<<ans<<endl;
				assert(ans!=-2);
				per[i]=ans;
			}
			
		}
		//exit(0);
	}while(DET);

	string ans="";
	rep(i,0,n)ans+='a';
	//trav(a,per)cout<<a<<endl;
	rep(i,0,n){
		ans[per[i]]=alphabet[i];
	}
	//cout<<ans<<endl;
	cout<<ans<<"\n";
	//vector<int> unk;
	//rep(i,0,n){
		//if(per[i]==-1)unk.push_back(i);
	//}
	//vector<int> newper(n);
	//do{
		//int ptr=0;
		//rep(i,0,n){
			//if(per[i]!=-1)newper[i]=per[i];
			//else newper[i]=unk[ptr],ptr++;
		//}
		//rep(i,0,n){
			//rep(j,0,n){
				//int u=i*j;
				
			//}
		//}
	//}while(next_permutation(unk.begin(),unk.end()));
	//rep(i,0,n)cout<<per[i]<<" ";
	//cout<<endl;
}

signed main() {
	//rep(i,47,48){
		//calc(i);
	//}
	rep(i,0,256)idx[i]=-1;
	rep(i,0,(int)alphabet.size())idx[(int)alphabet[i]]=i;
	ios::sync_with_stdio(0);
	cin.tie(0);
	int tt = 1;
	cin >> tt;
	while (tt--) solve();
	return 0;
}