#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define nl '\n'
int n, k;
string s;

int main() {
    int t; cin >> t;
    while(t--){
        cin >> n >> k;
        cin >> s;
        if(n == k) {
            cout << 0 << nl;
            continue;
        }
        if(0 == k) {
            cout << s << nl;
            continue;
        }
        map<int, int> mp;
        for(int i=1;i<(int)s.size() and k;++i){
            if(i and s[i]>s[i-1]){
                s.erase(s.begin()+i-1);
                // cerr << i << endl;
                k--;
                i-=2;
                continue;
            }
        }
        while(k--) s.pop_back();
        cout << s << nl;
    }
}