#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define nl '\n'
int main() {
    int t; cin >> t;
    while(t--){
        int n, k; cin >> n >> k;
        string s; cin >> s;
        map<int, int> vis;
        if(n == k) {
            cout << 0 << nl;
            continue;
        }
        map<char, vector<int>> mp;
        for(int i=0;i<n;++i) mp[s[i]].push_back(i);
        for(auto [x,y]: mp){
            if(!k) break;
            for(auto v: y) {
                vis[v] = 1;
                k--;
                if(!k) break;
            }
        }
        for(int i=0;i<n;++i) if(!vis[i]) cout << s[i];
        cout << nl;

    }
}