#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <list>
#include <string.h>
using namespace std;

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';

				printf("%c --> %c\n",a+97,b+97);
				adj[a][b] = 1;
				break;
			}
		}
	}
}

vector<int> seen, order;
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 c;
	cin >> c;
	while (c--) {
		int n;
		vector<string> words;

		cin >> n;
		for (int i=0; i<n; i++) {
			string t;
			cin >> t;
			words.push_back(t);
		}
		
		makeGraph(words);
		vector<int> ret = topologicalSort();

		for (int i=0; i<ret.size(); i++) {
			printf("%c",ret[i]+97);
			//printf("%d ",ret[i]);
		}

		cout << endl;
	}
}