#include<bits/stdc++.h>
using namespace std;
#define ll long long int 
int main()
{
    
    ll tc;
    cin>>tc;
    while(tc--)
    {
        ll n,k;
        cin>>n>>k;
        string s;
        cin>>s;
        string p,l;
        p=s;
        sort(p.begin(),p.end());
        reverse(p.begin(),p.end());
        map<char,ll>mp;
        for(int i=0;i<n-k;i++)
            l+=p[i];
        if(l.size()==0) cout<<0<<endl;
        else
            cout<<l<<endl;
    }
}