A_Ela_Sorting_Books.cpp

#include <bits/stdc++.h>
using namespace std;
 
typedef long long LL;
typedef pair<int, int> pii;
typedef pair<LL, LL> pll;
typedef pair<string, string> pss;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vii;
typedef vector<LL> vl;
typedef vector<vl> vvl;
#define FOR(a, b, c) for (int(a) = (b); (a) < (c); ++(a))
#define FORN(a, b, c) for (int(a) = (b); (a) <= (c); ++(a))
#define FORD(a, b, c) for (int(a) = (b); (a) >= (c); --(a))
#define FORSQ(a, b, c) for (int(a) = (b); (a) * (a) <= (c); ++(a))
#define FORC(a, b, c) for (char(a) = (b); (a) <= (c); ++(a))
#define FOREACH(a, b) for (auto&(a) : (b))
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define YES                          cout << "YES\n";
#define NO                           cout << "NO\n";
#define Yes                          cout << "Yes\n";
#define No                           cout << "No\n";
#define TC(t) while (t--)


void solve(){
    int n,k,flag,temp;
    string s;
    cin>>n>>k>>s;
        vector<LL>freq(26,0);
        FOR(i,0,n)
        freq[s[i]-'a']++;
        FOR(i,0,k)
        {
            temp=0;
            flag=1;
            FOR(j,0,26)
            {
                if(freq[j]==0)
                {
                    flag=0;
                    cout<<(char)(j+'a');
                    break;
                }
                freq[j]--;
                temp++;
                if(temp==n/k)
                break;
            }
            if(flag)
            cout<<(char)(n/k+'a');
        }
        cout<<"\n";
}
int main(){
    ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
  LL t;
  cin>>t;
  TC(t){
      solve();  
  }

}