#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <bitset>
using namespace std;
const int Maxn = 1010;
char s[Maxn], st[Maxn];
bitset <2000> f[Maxn];
int n, m;
int main (){
    int i, j, k;
    int T;
    scanf ( "%d", &T );
    while ( T -- ){
        scanf ( "%d%d", &n, &m );
        scanf ( "%s", s+1 );
        if ( n % 2 == 1 ){ printf ( "Impossible\n" ); continue; }
        f[n/2+1].reset ();
        f[n/2+1][0] = 1;
        for ( i = n/2; i >= 1; i -- ){
            f[i].reset ();
            if ( s[i] != s[i+n/2] ) f[i] = f[i] | (f[i+1]<<1);
            else f[i] = f[i] | f[i+1];
            f[i] = f[i] | (f[i+1]<<2);
        }
        if ( f[1][m] == 0 ){ printf ( "Impossible\n" ); continue; }
        for ( i = 1; i <= n/2; i ++ ){
            for ( j = 'a'; j <= 'z'; j ++ ){
                int ret = 0;
                if ( s[i] != j ) ret ++;
                if ( s[i+n/2] != j ) ret ++;
                if ( m-ret >= 0 ){
                    if ( f[i+1][m-ret] == 1 ){
                        st[i] = st[i+n/2] = j;
                        m -= ret;
                        break;
                    }
                }
            }
        }
        for ( i = 1; i <= n; i ++ ) printf ( "%c", st[i] );
        printf ( "\n" );
    }
    return 0;
}