#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define dbg(a,b,c,d) cerr<<a<<" "<<b<<" "<<c<<" "<<d<<endl;
#define kill(a) {cout<<a<<endl;continue;}
#define KILL(a) {cout<<a<<endl;return 0;}
#define debug cerr<<"Error Found"<<endl;
#define mem(a,b) memset(a,b,sizeof(a))
#define lcm(a, b) (a/__gcd(a,b))*b
#define w(t) cin>>t;while(t--)
#define pi 2 * acos(0.0)
#define endl "\n"
int t, cs = 0;
const int mxn = 2e5 + 3, mod = 1e9 + 7;
int32_t main()
{
fast
w(t)
{
string s;
int n, k;
cin >> n >> k >> s;
vector<char>v;
for(int i = 0; i < n; i++)
{
while(k and v.size() and v.back() < s[i])v.pop_back(), k--;
v.push_back(s[i]);
}
while(k--)v.pop_back();
int sz = v.size(), i = 0;
while(i < sz and v[i] == '0')i++;
string ans = "";
for(; i < sz; i++)ans += v[i];
if(ans.empty())ans = "0";
cout << ans << endl;
}
}