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

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

        // Find the best pair to concatenate
        for (int j=0; j<n-1; j++) {
            string concat1 = arr[j] + arr[j + 1];
            if (concat1>max_con){
                max_con=concat1;
                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;
}