#include <algorithm> //for min
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int C, N;
string tmp;
vector<string> word;
vector<vector<int> > adj;
vector<bool> visited;
vector<int> order;

void makeGraph(const vector<string>& words) {

    for (int j=1; j<words.size(); j++) {
        int i = j-1;
        int 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) {
    visited[here] = true;
    for (int i=0; i<26; i++)
        if (!visited[i] && adj[here][i] == 1)
            dfs(i);
    order.push_back(here);
}

int main() {
    C = 0;
    std::cin >> C;
    while(C--) {
        N = 0;
        cin >> N;
        word = vector<string>(sizeof(word), "");
        for (int i=0; i<N; i++) {
            cin >> tmp;
            word.push_back(tmp);
        }
        adj = vector<vector<int> >(26, vector<int>(26, 0));
        makeGraph(word);
        visited = vector<bool>(26, false);
        order.clear();
        for (int i=0; i<26; i++) {
            if (!visited[i]) dfs(i);
        }
        reverse(order.begin(), order.end());
        bool chk = false;
        for (int i=0; i<26; i++) {
            for (int j=i+1; j<26; j++) {
                if (adj[order[j]][order[i]] == 1 ) {
                    chk = true;
                    cout << "INVALID HYPOTHESIS";
                    break;
                }
            }
        }
        if (!chk) {
        for (int i=0; i<26; i++) {
            cout << (char)(order[i]+'a');
        }
        }
        cout << endl;
    }
}