#include<iostream>
#include<vector>
#include<string>
#include<algorithm>
using namespace std;

vector<int> seen,order;
vector<vector<int>> adj;

void makegraph(const vector<string>& words)
{
    adj=vector<vector<int>>(26,vector<int>(26,0));
    for(int j=1; j<words.size(); j++)
    {
        int i=j-1, len=min(words[i].size(),words[j].size());
        for(int k=0; k<len; k++){

            if(words[i][k]!=words[j][k]){
                int a=words[i][k]-'a';
                int b=words[j][k]-'a';
                adj[a][b]=1;
                break;

            }}
    }
}
void dfs(int here)
{
    seen[here]=1;
    for(int there=0; there<adj.size();there++){
    if(adj[here][there]&&!seen[there])dfs(there);}
        order.push_back(here);

}

vector<int> topologicalsort(){
    int n= adj.size();
    seen=vector<int>(n,0);
    order.clear();
    for(int i=0; i<n;i++) if(!seen[i]) dfs(i);
    reverse(order.begin(), order.end());

    for(int i=0; i<n; i++)
        for(int j=i+1; j<n; j++)
            if(adj[order[j]][order[i]])
                return vector<int>();
    return order;

}


int main()

{

    int cases;
    cin>>cases;

    for(int cc=0; cc<cases; cc++)
    {
        vector<string> words;
        int num;
        vector<int>a;
        cin>>num;

        for(int i=0; i<num; i++)
        {

            string s;
            cin>>s;
            words.push_back(s);

        }
        makegraph(words);
        a=topologicalsort();

        if(a.empty())
            cout<<"INVALID HYPOTHESIS";
        else
        {
            for(int n=0;n<adj.size(); n++)
                {
                    char c=a[n]+'a';
            cout<<c;}


        }



    }
}