#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int t;
  cin >> t;
  while (t--) {
    long long int n, k;
    cin >> n >> k;
    string s, s1;
    cin >> s;
    long long int a[27] = {0}, c = 0, d = 0;
    for (long long int i = 0; i < n; i++) {
      if (a[s[i] - 'a'] == 0) d++;
      a[s[i] - 'a']++;
    }
    s1 = s;
    sort(s1.begin(), s1.end());
    if (k == 1)
      cout << s1 << endl;
    else {
      long long int c1 = 0, c2 = 0;
      for (long long int i = 0; i < n; i++) {
        if (s1[i] == s1[0])
          c1++;
        else
          break;
      }
      if (c1 < k) {
        cout << s1[k - 1] << endl;
      } else if (c1 == k) {
        if (d == 1)
          cout << s1[0] << endl;
        else if (d == 2) {
          long long int c2 = n - c1;
          if (c2 % k == 0)
            c2 = c2 / k;
          else
            c2 = (c2 / k) + 1;
          cout << s1[0] << "";
          for (long long int i = 0; i < c2; i++) cout << s1[c1] << "";
          cout << endl;
        } else {
          cout << s1[0] << "";
          for (long long int i = c1; i < n; i++) {
            cout << s1[i] << "";
          }
          cout << endl;
        }
      } else {
        if (d == 1) {
          cout << s1[0] << "";
          long long int c2 = c1 - k;
          if (c2 % k == 0)
            c2 = c2 / k;
          else
            c2 = (c2 / k) + 1;
          for (long long int i = 0; i < c2; i++) cout << s1[0] << "";
          cout << endl;
        } else {
          for (long long int i = k - 1; i < n; i++) cout << s1[i] << "";
          cout << endl;
        }
      }
    }
  }
}
