#include<bits/stdc++.h>
  
using namespace std;  
const int MAXN = 100010;  
int n,k;  
int Paiming[MAXN+1],tmp[MAXN+1];  
int flag;  
bool comp_sa(int i, int j)  
{  
    if(Paiming[i] != Paiming[j])   
        return Paiming[i] < Paiming[j];  
    else{
        int ri = i+k <= n? Paiming[i+k] : -1;  
        int rj = j+k <= n? Paiming[j+k] : -1;  
        return ri < rj;     
    }  
}  
  
void calc_sa(string &S, int *sa) 
{  
    n = S.size();  
    for(int i = 0; i <= n; i++)  
    {  
        sa[i] = i;  
        Paiming[i] = i < n ? S[i] : -1;  
    }  
      
    for( k =1; k <= n; k *= 2)  
    {  
        sort(sa,sa+n+1,comp_sa);  
        tmp[sa[0]] = 0;  
        for(int i = 1; i <= n; i++)  
        {  
            tmp[sa[i]] = tmp[sa[i-1]] + (comp_sa(sa[i-1],sa[i]) ? 1: 0);  
        }  
        for(int i = 0; i <= n; i++)  
        {  
            Paiming[i] = tmp[i];  
        }  
    }  
}  
  
int SuffixArrayMatch(string &S, int *sa, string T)  
{  
    
    int lhs = 0, rhs = S.size();  
    while(rhs - lhs > 1)  
    {  
        int mid = (lhs + rhs)>>1;  
        if(S.compare(sa[mid],T.size(),T) < 0) lhs = mid;
        else rhs=mid;   
    }
    return S.compare(sa[rhs],T.size(),T) == 0;  
}  
int main()  
{  
    int t;
    ios::sync_with_stdio(false);
    cin>>t;        
    string s[100010],longs;    
    while(t--){
        int n,l=-1,p=-1,i; 
        cin>>n;
        memset(Paiming,0,sizeof Paiming);
        memset(tmp,0,sizeof tmp);
        for(i=0;i<n;i++){
            cin>>s[i];
            int len=s[i].size();
            if(len>l){
                l=len;
                longs=s[i];
                p=i;
            }
        }
        if(n==1){
            cout<<longs<<endl;
            continue;
        }   
        int *sa = new int[longs.size()+1];  
        calc_sa(longs,sa);
        for(i=0;i<n;i++){
            if(p==i)continue;    
            if(!SuffixArrayMatch(longs,sa,s[i]))
            break;  
        } 
        //delete [] sa;  
        sa = NULL; 
        if(i>=n){
            cout<<longs<<endl;
        }else {
            cout<<"No"<<endl;
        }      
    }   
}