#include<stdio.h>
#include<vector>
#include<string.h>
#include<string>
#include<stack>

using namespace std;

bool check[30][30];
bool visited[30];
stack<int> pathh;

bool DFS(int now, int path)
{
    visited[now] = true;
    path += (1 << now);

    for (int i = 0; i < 26; i++)
    {
        if (check[now][i] == 1 && (path &(1 << i))>0)
            return false;
        else if (check[now][i] == 1 && visited[i] == false)
            if (DFS(i, path)== false) return false;
    }
    pathh.push(now);
    return true;
}
int main()
{
    freopen("input.txt", "r", stdin);

    int t;
    scanf("%d", &t);
    while (t--)
    {
        for (int i = 0; i < 30; i++)
        {
            visited[i] = 0;
            for (int j = 0; j < 30; j++) check[i][j] = 0;
        }

        int n;
        scanf("%d", &n);

        char temp[1001][30];
        for (int i = 0; i < n; i++)
            scanf("%s", temp[i]);

        for (int i = 0; i < n - 1; i++)
        {
            int idx = 0;
            while (idx < strlen(temp[i]) && idx < strlen(temp[i + 1]))
            {
                if (temp[i][idx] == temp[i + 1][idx]) idx++;
                else
                {
                    check[temp[i][idx] - 'a'][temp[i + 1][idx] - 'a'] = 1;
                    break;
                }
            }
        }


        vector<int> canstart;

        for (int i = 0; i < 26; i++)
        {
            int j,l=0;
            for (j = 0; j < 26; j++)
            {
                if (check[i][j] == 1) l++;
                if (check[j][i] == 1) break;
            }
            if (j == 26 && l!=0) canstart.push_back(i);
        }

        if (canstart.empty()){
            printf("INVALID HYPOTHESIS\n");
            continue;
        }


        int i;

        for (i = 0; i < canstart.size(); i++)
            if (DFS(canstart[i], 0) == false) break;


        if (i != canstart.size()) {
            printf("INVALID HYPOTHESIS\n");
            continue;
        }

        while (!pathh.empty()) {
            printf("%c", pathh.top() + 'a'); pathh.pop();
        }
        for (int i = 0; i < 26; i++) if (!visited[i]) printf("%c", i + 'a');
        printf("\n");


    }
    return 0;
}