#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int t;
  cin >> t;
  while(t--) {
    int n,k;
    cin >> n >> k;
    vector<string> a(n);
    for(int i=0;i<n;i++){
      cin >> a[i];
    }   
    deque<string> ass;
    string nn="";
    string ans;
    for(int i=0;i<k+1;i++){
      ass.push_back(a[i]);
    }
    nn = max(nn,ans);
    for(auto as:ass){
      ans+=as;
    }
    //cout << ans <<"\n";
    ans.clear();
    for(int i=1;i<n-k;i++){
      ass.pop_front();
      ass.push_back(a[i+k]);
      for(auto as:ass){
        ans+=as;
      }
      //cout << ans <<"\n";
      nn = max(nn,ans);
      ans.clear();
      ans = "";
    }
    cout << nn <<"\n";
    // m = max(m,s);
    // cout << m <<"\n";
  }
}