#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define endl '\n'
#define ll long long
#define ull unsigned long long
#define ld long double
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define vi vector<ll>
#define maxn 500005
#define mod 1000000007
#define inf 1000000007
#define pii pair<ll,ll>
#define vii vector<pii>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define lcm(a,b) ((a*b)/__gcd(a,b));
#define srt(v) sort(v.begin(),v.end())
#define rsrt(v) sort(v.rbegin(),v.rend())
#define setbits(x) __builtin_popcountll(x)
#define rep(i, a, b) for(ll i = (a); i < (b); i++)
#define per(i, a, b) for(int i = (a); i >= (b); i--)
#define rev_str(str) reverse(str.begin(),str.end());
#define print(v) for(auto e:v) cout<<e<<" "; cout<<endl;
#define sum(a) (accumulate((a).begin(), (a).end(), 0LL))
#define printp(v) for(auto e:v) cout<<e.first<<" "<<e.second<<endl;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
bool sortByValue(const pair<int, int>& a, const pair<int, int>& b) {return a.second > b.second;}
void solve(){
ll n,k;cin>>n>>k;
string s;cin>>s;
map<char,ll>mp;
rep(i,0,n){
mp[s[i]]++;
}
//printp(mp);
if (n == 0){
cout << endl;
return;
}
if (n == k){
cout << 0 << endl;
return;
}
while(k--){
bool ok = false;
for (char i = '0'; i <= '9'; i++){
if (mp[i] > 0){
mp[i]--;
//cout<<i<<endl;
for (int j = 0; j < n; j++){
if (s[j] == i){
s[j] = 'x';
ok=true;
break;
}
}
}
if(ok){
break;
}
}
}
bool ashol = false;
string b="";
rep(i,0,n){
if(s[i]=='x'){
continue;
}
else{
//cout<<s[i];
b+=s[i];
}
}
ll cnt=0;
while(b[cnt]=='0'){
cnt++;
}
rep(i,cnt,sz(b)){
cout<<b[i];
}
cout<<endl;
}
int main() {
fast
ll t=1;cin>>t;
while (t--) {
solve();
}
return 0;
}