#include <bits/stdc++.h>
using namespace std;
#define esh29() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ll long long int
#define pb push_back
#define ep emplace_back
#define forn(i, n) for(int i = 0; i < int(n); i++)
#define form(i, n) for(int i = 1; i <= int(n); i++)
#define MOD 1000000007
#define w(t) int t; cin >> t; cin.ignore(); while (t--)

const double eps = 1e-9;
const double PI = acos(-1);
#define F first
#define S second
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define sz(x) (int)x.size()
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int,int> pii;
typedef pair<double, double> pdd;
typedef pair<ll, ll> pll;
typedef vector<pii> vii;
typedef vector<pll> vll;
typedef double dl;
ll gcd ( ll a, ll b ) { return __gcd ( a, b ); }
ll lcm ( ll a, ll b ) { return a * ( b / gcd ( a, b ) ); }

void solve() {
ll n,m,i,l;
cin>>n>>m;
if (n>0){
string a;
cin >> a;
if (n==m){
  cout<<0<<endl;
}
else {
   sort(rall(a));
   for (i=0; i<(n-m); i++){
    cout<<a[i];
   }
   cout<<endl;
}
}
else {
  cout<<endl;
}



}
int main() {
    esh29();
  w(t){
    solve();
   }
  
    return 0;
}