# include <stdio.h>
# include <algorithm>
# include <cstring>
using namespace std;
int next[100];
char pat[100];
char a[100][100];
int ma;
int lenp;
int n;
void Getnext()
{
    int i=0,j=-1;
    next[0]=-1;
    while(i<=lenp)
    {
        if(j==-1||pat[j]==pat[i])
            j++,i++,next[i]=j;
        else
            j=next[j];
    }
}
void kmp()
{
    int k,i,j,m;
    Getnext();
    ma=100;
    for(k=1; k<n; k++) //匹配剩下的字符串
    {
        i=0;
        j=0;
        m=0;
        while(i<60&&j<lenp)//每个匹配的最长长度
        {
            if(j==-1||pat[j]==a[k][i])
            {
                i++;
                j++;
            }
            else
                j=next[j];
            if(j>m)
                m=j;
        }
        if(ma>m)//都要匹配中的最短
            ma=m;
    }
}
int main()
{
    int  t, i;
    char result[100];
    char tmp[100];
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d",&n);
        for(i=0; i<n; i++)
            scanf("%s",a[i]);
        int ans=0;
        for(i=0; i<=59; i++)
        {
            strcpy(pat,a[0]+i);// 枚举第一个串的所有后缀串,strcpy
            lenp=60-i;
            kmp();
            if(ma>ans)
            {
                ans=ma;
                strncpy(result,a[0]+i,ans);//strcnpy
                result[ans]='\0';
            }
            else if(ma==ans)//输出字典序最小
            {
                strncpy(tmp,a[0]+i,ans);
                tmp[ans]='\0';
                if(strcmp(tmp,result)<0)
                    strcpy(result,tmp);
            }
        }
        if(ans>=3)
            printf("%s\n",result);
        else
            printf("no significant commonalities\n");
    }
    return 0;
}