#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
void print(vector<long long> a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i] << ' ';
  }
  cout << endl;
}
vector<long long> inp(int n) {
  vector<long long> a;
  long long x;
  for (int i = 0; i < n; i++) {
    cin >> x;
    a.push_back(x);
  }
  return a;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t, n, i, j, k, l, c, c1, cnt, flag, m1, m2, maxi, mini, x, y, z;
  vector<long long> a, b;
  string s, s1, s2;
  cin >> t;
  map<char, long long> m;
  while (t--) {
    cin >> n >> k;
    cin >> s;
    for (int i = 0; i < n; i++) {
      m[s[i]]++;
    }
    map<char, long long>::iterator itr;
    itr = m.begin();
    if (m.size() == 1) {
      char ch = itr->first;
      x = (itr->second) / k;
      if ((itr->second) % k != 0) {
        x++;
      }
      string b(x, ch);
      cout << b << endl;
    } else if (m.size() == 2 && itr->second == k) {
      cout << itr->first;
      itr++;
      char ch = itr->first;
      x = (itr->second) / k;
      if ((itr->second) % k != 0) {
        x++;
      }
      string b(x, ch);
      cout << b << endl;
    } else {
      if (itr->second < k) {
        c = 0;
        for (itr = m.begin(); itr != m.end(); itr++) {
          c = c + itr->second;
          if (c >= k) {
            cout << itr->first << endl;
            break;
          }
        }
      } else {
        itr->second = itr->second - (k - 1);
        for (itr = m.begin(); itr != m.end(); itr++) {
          for (j = 0; j < itr->second; j++) {
            cout << itr->first;
          }
        }
        cout << endl;
      }
    }
    m.clear();
  }
}
