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

string solve(int n, int k, vector<string>& arr) {
    for (int i = 0; i<k && n>1; ++i) {
        int pos = -1;
        string max_con ="";

        for (int j = 0; j < n - 1; ++j) {
            string concat = arr[j] + arr[j + 1];
            if (concat >= max_con) {
                max_con = concat;
                pos = j;
            }
        }

        arr[pos] = max_con;
        arr.erase(arr.begin() + pos + 1);
        --n;
    }

    return *max_element(arr.begin(), arr.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;
}