#include <iostream>
#include <string>
#include <queue>
#include <vector>
using namespace std;
inline int min(int a, int b) {
    if(a<b) return a;
    return b;
}
string str[1111];
void task() {
    int n;
    int graph[300]={0}; // is alphabet is in graph
    int incoming[300]={0}; // prerequisites
    int edge[100][100]={0};
    cin>>n;
    int i,j,k;
    for(i=1;i<=n;i++) {
        cin>>str[i];
    }

    for(j=2;j<=n;j++) {
            i=j-1;
            int l = min(str[i].size(), str[j].size());
            for(k=0;k<l;k++) {
                if(str[i][k] != str[j][k]) {
                    if(!edge[str[i][k]-'a'][str[j][k]-'a']) {
                        incoming[str[j][k] - 'a']++;
                        edge[str[i][k] - 'a'][str[j][k] - 'a'] = 1;
                        graph[str[i][k] - 'a'] = 1;
                        graph[str[j][k] - 'a'] = 1;
                    }
                    break;
                }
            }
    }

    int cnt=0; // how many are in graph
    queue<int> head;
    for(i=0;i<26;i++) {
        if(graph[i]) {
            cnt++;
            if(!incoming[i]) {
                head.push(i);
            }
        }
    }

    vector<int> ans;
    int vis[100]={0};
    int cnt2=0;
    while(!head.empty()) {
        int tt = head.front();
        head.pop();
        queue<int> qu;
        qu.push(tt);
        vis[tt]=1;
        while(!qu.empty()) {
            cnt2++;
            int t = qu.front();
            ans.push_back(t);
            qu.pop();
            for(i=0;i<26;i++) {
                if(edge[t][i]) {
                    incoming[i]--;
                    if(incoming[i]==0 && !vis[i]) {
                        vis[i]=1;
                        qu.push(i);
                    } else if(incoming[i] == 0 && vis[i]) {
                        cout<<"INVALID HYPOTHESIS"<<endl;
                        return;
                    }
                }
            }
        }
    }

    if(cnt == cnt2) {
        for(i=0;i<ans.size();i++) {
            printf("%c",(ans[i]+'a'));
        }
        for(i=0;i<26;i++) {
            if(!graph[i]) printf("%c",i+'a');
        }
        cout<<endl;
    } else {
        cout<<"INVALID HYPOTHESIS"<<endl;
    }
}
int main() {
    cin.sync_with_stdio(false);
    cout.sync_with_stdio(false);
    int c;
    cin>>c;
    while(c--) {
        task();
    }
}