#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;
        if(N==0)
        {
            cout<<0<<"\n";
        }
        else
        {
            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";
        }
    }

    return 0;
}