#include<iostream>
#include<cstdio>
#include<string>
using namespace std;
int main()
{
int t,n,i,j,l,flg;
char ch[100];
string str[20],s;
scanf("%d",&t);
while(t--)
{
string ans;
scanf("%d",&n);
for(i=0; i<n; i++)
{
scanf("%s",&ch);
str[i]=ch;
}
for(l=3;l<=60;l++)
for(i=0;i<=60-l;i++)
{
flg=1;
s=str[0].substr(i,l);//从str下标为0中截取从i开始长度为l的子串
for(j=1;j<n;j++)
if(str[j].find(s)==string::npos)
{
flg=0;
break;
}
if(flg&&s.size()>ans.size())//如果s长度大于前一个记录的长度,则用s替换ans
ans=s;
if(flg&&s.size()==ans.size()&&s<ans)//如果相等,则比较字典大小,取小
ans=s;
}
if(!ans.empty()) //如果ans不为空
cout<<ans<<endl;
else cout<<"no significant commonalities"<<endl;
}
return 0;
}