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

#define pb push_back
#define all(v) v.begin(),v.end()
#define endl '\n'
#define int long long
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)

string concatenate(string a, string b) {
    return a + b;
}

void solve() {
    int n, k;
    cin >> n >> k;
    vector<string> arr(n);
    for(int i = 0; i < n; i++) {
        cin >> arr[i];
    }

    for(int i = 0; i < k; i++) {
        int max_index = -1;
        string max_concat = "";
        
        for(int j = 0; j < arr.size() - 1; j++) {
            string concat1 = concatenate(arr[j], arr[j + 1]);
            if (concat1 > max_concat) {
                max_concat = concat1;
                max_index = j;
            }
        }
        
        if (max_index == -1) break; 

        arr[max_index] = max_concat;
        arr.erase(arr.begin() + max_index + 1);
    }
    string max_element_in_array = *max_element(arr.begin(), arr.end());
    cout << max_element_in_array << endl;
}

signed main() {
    fastio;
    int t;
    cin >> t;
    while (t--) {
        solve();
    }
}