from collections import defaultdict
import sys
input = sys.stdin.readline
def inp():
return(int(input()))
def inlt():
return(list(map(int,input().split())))
def insr():
s = input()
return(list(s[:len(s) - 1]))
def invr():
return(map(int,input().split()))
def compress(root, x):
if root[x] == x:
return x
root[x] = compress(root, root[x])
return root[x]
t = inp()
for _ in range(t):
n, k = inlt()
_dict = defaultdict(list)
s = input()
if s[-1] == '\n':
s = s[:len(s) - 1]
s = list(s)
root = [i for i in range(26)]
index = 0
while k > 0 and index < len(s):
ch = s[index]
idx = ord(ch) - ord('a')
compress(root, idx)
if root[idx] > 0:
root[idx] = idx - 1
s[index] = chr(root[idx] + ord('a'))
k -= 1
else:
index += 1
ans = ''
for ch in s:
idx = ord(ch) - ord('a')
changed = compress(root, idx)
ans += chr(changed + ord('a'))
print(ans)
#include<bits/stdc++.h>
using namespace std;
int n,k;
string s;
void solve()
{
cin>>n>>k>>s;
int now=0,tw=-1,m;
for(int i=0;i<n;i++)
{
if(s[i]-'a'<=k)
{
now=max(now,s[i]-'a');
s[i]='a';
}
else
{
if(tw==-1)
{
tw=s[i]-'a';
m=s[i]-(k-now)-'a';
}
k=now;
if(s[i]-'a'<=tw && s[i]-'a'>=m)
{
s[i]='a'+m;
}
}
}
cout<<s<<endl;
}
signed main()
{
int _=1;
cin>>_;
while(_--)
{
solve();
}
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