#include<bits/stdc++.h>
using namespace std;

#define ll long long;

int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int N,k;
        cin>>N>>k;
        string s;
        cin>>s;
        sort(s.begin(),s.end());
        reverse(s.begin(),s.end());
        if(N-k==0)
        {
            cout<<'0'<<"\n";
        }
        else
        {
        for(int i=0;i<N-k;i++)
        {
            cout<<s[i];
        }
        cout<<"\n";
        }
    }
}