#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")
std::pair<int, int> DR[] = {{-1, 0}, {0, 1},   {1, 0}, {0, -1},
                            {-1, 1}, {-1, -1}, {1, 1}, {1, -1}};
using namespace std;
const long long mod = 998244353;
long long nop = 70;
vector<int> primes(nop + 1, 1);
void sieve() {
  primes[1] = primes[0] = 0;
  for (long long i = 2; i * i <= nop; i++) {
    if (primes[i]) {
      for (long long j = i * i; j <= nop; j += i) {
        primes[j] = 0;
      }
    }
  }
}
long long gcd(long long a, long long b) {
  if (b > a) swap(b, a);
  if (b == 0) return a;
  return gcd(b, a % b);
}
void extgcd(long long a, long long b, long long& x, long long& y) {
  if (a == 0) {
    x = a;
    y = b;
    return;
  }
  extgcd(b % a, a, x, y);
  long long y1 = y, x1 = x;
  x = y1 - (b / a) * x1;
  y = x1;
}
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
long long bexp(long long num, long long e) {
  long long ans = 1;
  while (e > 0) {
    if (e & 1) {
      ans *= num;
    }
    num *= num;
    e >>= 1;
  }
  return ans;
}
long long mexp(long long num, long long e) {
  long long ans = 1;
  while (e > 0) {
    if (e & 1) {
      ans = (ans * num) % mod;
    }
    num *= num;
    num %= mod;
    e >>= 1;
  }
  return ans % mod;
}
long long modinv(long long a) {
  a %= mod;
  return mexp(a, mod - 2);
}
void solve() {
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  sort(s.begin(), s.end());
  vector<long long> cnt(26, 0);
  for (long long i = 0; i < (n); i++) {
    cnt[s[i] - 'a']++;
  }
  int first = -1, last = -1, nod = 0;
  for (long long i = 0; i < (26); i++) {
    if (cnt[i]) {
      nod++;
      if (first == -1) first = i;
      last = i;
    }
  }
  if (cnt[first] < k) {
    cout << s[k - 1] << "\n";
    return;
  }
  string worst = "";
  worst += char('a' + first);
  if (nod >= 3 || (nod == 2 && cnt[first] > k)) {
    for (long long i = k; i < n; i++) {
      worst += s[i];
    }
  } else {
    for (long long i = k; i < n; i++) {
      worst += s[i];
      i += (k - 1);
    }
  }
  cout << worst << "\n";
}
int main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  int t = 1;
  cin >> t;
  for (long long T = 1; T <= (t); T++) {
    solve();
  }
  return 0;
}
