#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <set>
#include <cstdlib>
#include <queue>
#include <sstream>
#include <map>
#include <iomanip>
#include <math.h>
#include <numeric>
#include <stdexcept>
#include <unordered_set>

using namespace std;

//#pragma GCC optimize("O3")
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("Ofast", "O3", "unroll-loops")
//#pragma GCC optimize("sse,sse2,sse3,sse4")


typedef long long ll;
typedef unsigned long long ull;
#define pb push_back
#define mp make_pair
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()


signed main() {

    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    int q;
    cin >> q;
    while (q--) {
        string s;
        int n, k;
        map<char, int> m;
        cin >> n >> k >> s;
        for (auto ch : s) m[ch]++;
        string res;
        while (!m.empty()) {
            char ch = m.begin()->first;
            int p = m.begin()->second;
            if (p >= k) {
                res += ch;
                if (p == k) m.erase(ch);
                else m.begin()->second -= k;
                continue;
            }
            else if (m.size() == 1) {
                if (ch != res.back()) {
                    sort(all(s));
                    res = string(s.begin() + k - 1, s.end());
                    break;
                }
                else {
                    res += ch;
                    break;
                }
            }
            else if (res.size() == 0) {
                int count = 0;
                vector<char> chs;
                while (count < k && !m.empty()) {
                    chs.push_back(m.begin()->first);
                    m.begin()->second--;
                    if (m.begin()->second == 0) m.erase(m.begin());
                    ++count;
                }
                sort(all(chs));
                res += chs.back();
                break;
            }
            else {
                while (!m.empty()) {
                    res.push_back(m.begin()->first);
                    m.begin()->second--;
                    if (m.begin()->second == 0) m.erase(m.begin());
                }
                break;
            }
        }
        cout << res << "\n";
    }
}