#include<bits/stdc++.h>
#define ll long long
#define endl '\n'
using namespace std;
int main()
{
    ios_base:: sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    ll t;
    cin>>t;

    while(t--){
        ll n, k;
        cin>>n>>k;
        string st;
        cin>>st;

       if(n==k){
        cout<<'0'<<endl;
       }
       else{
        ll arr[n];
        vector<ll>arr2;
        for(ll i=0; i<st.size(); i++){
            ll tm= st[i]-'0';
            arr[i]= tm;
            arr2.push_back(tm);
        }

         sort(arr, arr+n);
        for(ll i=n-1; i>=k; i--){
            cout<<arr[i];
        }
        cout<<endl;

       }

    }






    return 0;
}