#include<bits/stdc++.h>
using namespace std;
int main()
{
int T;
cin>>T;
while(T--)
{
string sr;
int num,p;
cin>> num>> p;
cin>>sr;
int i=0;
int j=num-1;
vector<bool> swap (num);
while(i<j && p>0)
{
while (sr[i]=='a' && i<j)
{
i++;
}
while(sr[j]=='b' && i<j)
{
j--;
}
if(i<j)
{
char temp =sr[i];
sr[i]=sr[j];
sr[j]=temp;
swap[j]=true;
p--;
}
}
for(int i=0;i<num && p>0;i++)
{
if(sr[i]=='b' && swap[i])
{
sr[i]='a';
p--;
}
else if(sr[i]=='b' && !swap[i] && p>1)
{
sr[i]='a';
p -=2;
}
}
cout<<sr<<endl;
}
return 0;
}
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