#include <bits/stdc++.h>
using namespace std;

string solve(int n, int k, vector<string>& arr) {
    
    vector<string> current = arr;

    for (int operations = 0; operations < k && n > 1; ++operations) {
        string max_concat = "";
        int pos = -1;

      
        for (int i = 0; i < n - 1; ++i) {
            string concat = current[i] + current[i + 1];
            if (concat > max_concat) {
                max_concat = concat;
                pos = i;
            }
        }

      
        if (pos != -1) {
            current[pos] = max_concat; 
            current.erase(current.begin() + pos + 1);
            --n; 
        }
    }

    return *max_element(current.begin(), current.end());
}

int main() {
    int t;
    cin >> t;

    while (t--) {
        int n, k;
        cin >> n >> k;
        vector<string> str(n);

        for (int i = 0; i < n; ++i) {
            cin >> str[i];
        }

        cout << solve(n, k, str) << endl;
    }

    return 0;
}