#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define nl '\n'
int main() {
    int t; cin >> t;
    while(t--){
        ll n,k; string s, t, ans; cin >> n >> k >> s;
        t = s;
        ans = s;
        sort(t.begin(), t.end());
        map<char, set<int>> mp, lagbe;
        for(int i=0;i<n;++i) mp[s[i]].insert(i);
        for(int i=0;i<n;++i) lagbe[t[i]].insert(i);
        for(auto [c, v]: lagbe){
            int mx = *v.rbegin();
            while(mp[c].size() and *mp[c].begin() <=mx){
                mp[c].erase(*mp[c].begin());
            }
            vector<int> sw;
            vector<char> chr;
            int cnt = 0;
            while(mp[c].size() and k){
                sw.push_back(*mp[c].rbegin());
                mp[c].erase(*mp[c].rbegin());
                --k;
                cnt++;
            }
            for(int i=*v.begin();i<=*v.rbegin() and cnt;++i)
                if(s[i]!=c)
                    chr.push_back(s[i]),mp[s[i]].erase(i),cnt--, s[i] = c;
            sort(sw.begin(), sw.end());
            sort(chr.begin(), chr.end());
            for(int i=0;i<sw.size();++i) {
                s[sw[i]] = chr[i];
                mp[s[sw[i]]].insert(sw[i]);
            }
            // cerr << s << nl;
        }
        cout << s << nl;
    }
}