#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  sort(s.begin(), s.end());
  if (s[0] != s[k - 1]) {
    cout << s[k - 1] << endl;
    return;
  }
  cout << s[0];
  if (s[k] != s[n - 1]) {
    for (int i = k; i < n; i++) cout << s[i];
  } else {
    for (int i = 0; i < (n - 1) / k; i++) cout << s[n - 1];
  }
  cout << endl;
}
int main() {
  int t;
  cin >> t;
  while (t--) {
    solve();
  }
}
