#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
bool func(string s1, string s2)
{
    if (s1.length() != s2.length()) return s1.length() < s2.length();
    else return s1 < s2;
}
vector <string> word;
vector <string> result;
int main()
{
    int loop;
    cin >> loop;
    for (int l = 0; l < loop; l++)
    {
        word.clear();
        result.clear();
        int n, m;
        cin >> n >> m;
        for (int i = 0; i < n; i++)
        {
            string temp;
            cin >> temp;
            for (int j = 0; j < temp.length(); j++)
            {
                word.push_back(temp.substr(0, j + 1));
            }
        }
        sort(word.begin(), word.end(), func);
        int len = 0;
        for (int i = 0; i < (int)word.size(); i++)
        {
            int maxnum = 0;
            int count = 1;
            result.push_back(word[i]);
            while (true)
            {
                if (i >= (int)word.size() - 1)
                    break;
                if (word[i].length() != word[i + 1].length())
                    break;
                if (word[i] == word[i + 1])
                    count++;
                else
                    count = 1;
                if (count > maxnum)
                {
                    maxnum = count;
                    result.pop_back();
                    result.push_back(word[i]);
                }
                i++;
            }
            len++;
        }
        for (int i = 0; i < len; i++)
            cout << result[i] << endl;
    }
}