#include<bits/stdc++.h>
using namespace std;
#define ll long long int 
#define nl "\n"
#define all(x) (x).begin(), (x).end()
#define Yes cout<<"Yes"<<"\n";
#define No  cout<<"No"<<"\n"; 
using namespace std;
string concatenate(const string &a, const string &b) {
    return a + b;
}
string findMaxValue(vector<string> arr, int K) {
    while (K > 0) {
        int n = arr.size();
        int best_i = -1;
        string best_val;
        for (int i = 0; i < n - 1; i++) {
            string combined = concatenate(arr[i], arr[i + 1]);
            if (combined > best_val) {
                best_val = combined;
                best_i = i;
            }
        }
        if (best_i != -1) {
            arr[best_i] = best_val;
            arr.erase(arr.begin() + best_i + 1);
            --K;
        } else {
            break;
        }
    }
    return *max_element(all(arr));
}

int main() {
    int T;
    cin >> T;
    
    while (T--) {
        int N, K;
        cin >> N >> K;
        
        vector<string> arr(N);
        for (int i = 0; i < N; i++) {
            cin >> arr[i];
        }
        cout << findMaxValue(arr, K) << nl;
    }
    
    return 0;
}