#include<bits/stdc++.h>
using namespace std;
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());
        s.erase(n-k);
        if(s.length()==0){
            cout<<0<<"\n";
        }
        else{
            cout<<s<<"\n";
        }
    }
}