#include<bits/stdc++.h>
using namespace std;
int main()
{
    int t;
    cin>>t;
    while(t--){
        unsigned int t1,k;
        cin>>t1>>k;
        string s,s1,s2;
        cin>>s;
        ///orginal string s1
        s1=s;
        vector<int> v;
        ///after swaping s
        if(s.size()==k){
            cout<<0<<endl;
        }
        else{
            sort(s.rbegin(), s.rend());
        int sz=s.size()-k;
        for(int i=0; i<s1.size(); i++){
            for(int j=0; j<sz; j++){
                if(s1[i]==s[j]){
                        int d= s1[i]-'0';

                    v.push_back(d);
                    if(s1[i]=s[j]){
                        break;
                    }

                }

            }
        }

        for(int i=0; i<sz; i++){
            cout<<v[i];
        }
        cout<<endl;
        }

    }
    return 0;
}