#include <bits/stdc++.h>
using namespace std;
void read() { return; }
void print() { return; }
template <typename T1, typename... T2>
void read(T1 &x, T2 &...args) {
  ((cin >> x), read(args...));
}
template <typename T1, typename... T2>
void print(T1 x, T2... args) {
  ((cout << x << ' '), print(args...));
}
template <typename T>
void readArr(T &lst, int x, int y) {
  for (int i = (int)x; i < (int)y; ++i) cin >> lst[i];
}
template <typename T>
void printArr(T &lst, int x, int y) {
  for (int i = (int)x; i < (int)y; ++i) cout << lst[i] << ' ';
}
const int N = 1e6;
const int INF = 1e9;
const int MOD = 1e9 + 7;
int countBits(long long number) { return (int)log2(number) + 1; }
long long add(long long x, long long y) { return (x + y) % MOD; }
long long sub(long long x, long long y) { return (x - y + MOD) % MOD; }
long long mul(long long x, long long y) { return (x * 1ll * y) % MOD; }
long long inv(long long p, long long q) {
  long long expo = MOD - 2;
  while (expo) {
    if (expo & 1) p = mul(p, q);
    q = mul(q, q), expo >>= 1;
  }
  return p;
}
long long power(long long x, long long y) {
  if (y == 0)
    return 1;
  else if (y % 2 == 0) {
    long long tmp = power(x, y / 2);
    return mul(tmp, tmp);
  } else
    return mul(x, power(x, y - 1));
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a %= b);
}
void solveTestCase() {
  int n, k;
  read(n, k);
  string s;
  read(s);
  sort(s.begin(), s.end());
  if (k == 1)
    print(s);
  else if (s[0] != s[k - 1])
    print(s[k - 1]);
  else {
    int i = k + 1;
    while (i < n && s[i - 1] == s[i]) i++;
    if (i == n) {
      string ans = "";
      ans += s[k - 1];
      int x = (n - 1) / k;
      while (x--) ans += s[k];
      print(ans);
    } else
      print(s.substr(k - 1, n - k + 1));
  }
  cout << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int t = 1;
  cin >> t;
  while (t--) solveTestCase();
  return 0;
}
