#include<bits/stdc++.h>
#define ll unsigned long long
using namespace std;


int main(){
    ll t;
    cin>>t;
    while(t--){
        int n,k;
        cin >>n>>k;
        string num;
        cin >>num;
        if(num.length()==k){
            cout<<"0"<<endl;
        }
        else{
        sort(num.rbegin(),num.rend());

        int sz=num.size();


        for(int i =0;i<sz-k;i++){
            cout<<num[i];
        }
        cout<<endl;
        }
    }
    return 0;
}