#include <iostream>
#include <cstdio>
#include <vector>
#include <list>
#include <cstring>
#include <algorithm>

using namespace std;

#define ALPHABET_CNT 26

bool visited[ALPHABET_CNT];
vector<int> adj[ALPHABET_CNT];

void dfs(int idx, list<int>& path)
{
    visited[idx] = true;
    path.push_back(idx);
    for ( int i = 0; i < adj[idx].size(); ++i )
    {
        int there = adj[idx][i];
        if ( !visited[there] )
            dfs(there, path);
    }
}

list<int> getAlphabetOrder()
{
    list<int> orders;

    for ( int i = 0; i < ALPHABET_CNT; i++ )
    {
        if ( !visited[i] && !adj[i].empty() )
        {
            list<int> order;
            dfs(i, order);
            orders.splice(orders.begin(), order);
        }
    }

    for ( list<int>::iterator itor = orders.begin(); itor != orders.end(); ++itor )
        for ( list<int>::iterator itor2 = orders.begin(); itor2 != itor; ++itor2 )
            if ( find( adj[*itor].begin(), adj[*itor].end(), *itor2) != adj[*itor].end() )
                return list<int>();

    for ( int i = 0; i < ALPHABET_CNT; i++ )
        if ( !visited[i] )
            orders.push_back(i);

    return orders;
}

// a < b
void getGraph(string a, string b)
{
    string::iterator itA = a.begin();
    string::iterator itB = b.begin();

    while ( itA != a.end() && itB != b.end() )
    {
        if ( *itA != *itB )
        {
            adj[*itA-'a'].push_back(*itB-'a');
            break;
        }
        itA++;
        itB++;
    }
}

int main()
{
    int T;
    cin >> T;
    while ( T-- )
    {
        for ( int i = 0; i < ALPHABET_CNT; i++ )
        {
            visited[i] = false;
            adj[i].clear();
        }

        int N;
        cin >> N;

        string first;
        cin >> first;
        for ( int i = 0; i < N-1; i++ )
        {
            string next;
            cin >> next;
            getGraph(first, next);
            first = next;
        }
        list<int> result = getAlphabetOrder();

        if ( result.empty() )
            cout << "INVALID HYPOTHESIS" << endl;
        else
        {
            for ( list<int>::iterator itor = result.begin(); itor != result.end(); ++itor)
                printf("%c", *itor + 'a');
            cout << endl;
        }
    }

    return 0;
}