#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#include<cmath>
#include<cstdlib>
#include<bitset>
//#include<ctime>
#define ll long long
#define ull unsigned long long
#define ui unsigned int
#define d double
//#define ld long double
using namespace std;
const int maxn=1000233,inf=1002333333;
bitset<1001>f[501];
int mp[26][26][26];
int i,j,k,n,m;
char s[1013],S[505];

int ra,fh;char rx;
inline int read(){
    rx=getchar(),ra=0,fh=1;
    while((rx<'0'||rx>'9')&&rx!='-')rx=getchar();
    if(rx=='-')fh=-1,rx=getchar();
    while(rx>='0'&&rx<='9')ra=ra*10+rx-48,rx=getchar();return ra*fh;
}

inline int abs(int x){return x<0?-x:x;}
int main(){
    for(i=0;i<26;i++)for(j=0;j<26;j++)for(k=0;k<26;k++)mp[i][j][k]=(i!=k)+(j!=k);
    
    for(int T=read();T;T--){
        n=read(),m=read();
        scanf("%s",s+1);int n2=n/2,c1,c2;
        
        f[n2+1].reset(),f[n2+1][0]=1;
        for(i=n2;i;i--){
            f[i].reset(),c1=s[i]-'a',c2=s[i+n2]-'a';
            //for(j=0;j<26;j++)f[i]|=f[i+1]<<mp[c1][c2][j];
            if(c1==c2)f[i]|=f[i+1]<<2,f[i]|=f[i+1];
            else f[i]|=f[i+1]<<1,f[i]|=f[i+1]<<2;
        }
        if(!f[1][m]){puts("Impossible");continue;}
        
        int rest=m;
        for(i=1;i<=n2;i++){
            c1=s[i]-'a',c2=s[i+n2]-'a';
            for(j=0;j<26;j++)if((k=rest-mp[c1][c2][j])>=0&&f[i+1][k])
                {putchar(j+'a'),S[i]=j+'a',rest=k;break;}
        }
        for(i=1;i<=n2;i++)putchar(S[i]);puts("");
    }
}