#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;
        vector<char>st;
        int rem=k;
        for(char c:s){
            while(!st.empty()&&rem>0&&st.back()<c){
                st.pop_back();
                rem--;
            }
            st.push_back(c);
        }
        while(rem>0){
            st.pop_back();
            rem--;
        }
        if(n==k){
            cout<<0<<"\n";
        }
        else{
        for(char c:st){
            cout<<c;
        }
        cout<<"\n";
        }
    }
}