/*
 *   Copyright (c) 2024 Emon Thakur
 *   All rights reserved.
 */
#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
using minheap = priority_queue<long long, vector<long long>, greater<long long>>;
typedef tree<int, null_type, greater_equal<int>, rb_tree_tag, tree_order_statistics_node_update> pbds; // find_by_order, order_of_key

#define ll long long
#define ld long double
#define MOD 1000000007
#define pie 2*(acos(0.0))
#define yes cout<<"YES\n"
#define no cout<<"NO\n"
#define pb push_back
#define endl '\n'
#define lcm(a,b) (a*b)/(__gcd<ll>(a,b))
#define print(v) for(auto e:v) cout<<e<<" "; cout<<endl;
#define printp(v) for(auto e:v) cout<<e.first<<" "<<e.second<<endl;
#define srt(v) sort(v.begin(),v.end())
#define rsrt(v) sort(v.rbegin(),v.rend())
#define life_is_a_race ios::sync_with_stdio(false); cin.tie(nullptr);

void solve(int tc)
{
    //cout<<"Case "<<tc<<": ";
    ll n, m;
    string s1, s2;
    cin >> n >> m >> s1;
    s2 = s1;
    sort(s2.begin(), s2.end());
    if(n <= m + 1) {
        cout << s2 << endl;
        return;
    }
    map<char, set<int, greater<int>>> mp;
    for(int i = 0; i < n; i++) {
        mp[s1[i]].insert(i);
    }
    char last = '0';
    priority_queue<char> pq;
    for(int i = 0; i < n && m > 0; i++) {
        if(last != s2[i]) {
            for(auto &j: mp[last]) {
                if(pq.size() == 0) break;
                s1[j] = pq.top();
                mp[s1[j]].insert(j);
                pq.pop();
            }
            last = s2[i];
        }
        if(s1[i] == s2[i]) {
            mp[s1[i]].erase(i);
            continue;
        }
        pq.push(s1[i]);
        mp[s1[i]].erase(i);
        s1[i] = s2[i];
        --m;
    }
    if(pq.size()) {
        for(auto &j: mp[last]) {
            if(pq.size() == 0) break;
            s1[j] = pq.top();
            pq.pop();
        }
    }
    cout << s1 << endl;
}
int main()
{
    life_is_a_race
    int t=1; 
    cin>>t;
    for(int i=1;i<=t;i++) solve(i);
}