#include<cstdio>
#include<cstring>
using namespace std;
int t,m,n,i,j,temp,now;
char s[2010],c[2010],ans[2010][2010];
bool f[2010][2010];
int main()
{
    scanf("%d",&t);
    while (t--)
    {
        scanf("%d%d",&m,&n);
        scanf("%s",s);
        memset(f,0,sizeof(f));
        f[m/2][0]=1;
        for (i=m/2-1;i>=0;i--)
          for (j=0;j<=n;j++)
          {
              if (f[i+1][j])
              {
                  if (s[i]==s[i+m/2]) 
                {
                    if (s[i]=='a')
                    {
                        ans[i][j]='a';f[i][j]=1;
                        if (!f[i][j+2]) {f[i][j+2]=1;ans[i][j+2]='b';}
                        else if ('b'<ans[i][j+2]) ans[i][j+2]='b';
                    }
                    else
                    {
                        f[i][j+2]=1;ans[i][j+2]='a';
                        if (!f[i][j]) {f[i][j]=1;ans[i][j]=s[i];}
                        else if (s[i]<ans[i][j]) ans[i][j]=s[i];
                    }
                }
                else
                {
                    char ch=s[i];if (ch>s[i+m/2]) ch=s[i+m/2];
                    if (!f[i][j+1]) {f[i][j+1]=1;ans[i][j+1]=ch;}
                    else if (ch<ans[i][j+1]) ans[i][j+1]=ch;
                    if (s[i]!='a'&&s[i+m/2]!='a') ch='a';
                    else if (s[i]!='b'&&s[i+m/2]!='b') ch='b';
                    else ch='c';
                    if (!f[i][j+2]) {f[i][j+2]=1;ans[i][j+2]=ch;}
                    else if (ch<ans[i][j+2]) ans[i][j+2]=ch;
                }
              }
          }
        if (!f[0][n]) {printf("Impossible\n");continue;}
        now=n;
        for (i=0;i<m/2;i++)
        {
            c[i]=ans[i][now];
            temp=2;
            if (c[i]==s[i]) temp--;
            if (c[i]==s[i+m/2]) temp--;
            now-=temp;
        }
        for (i=0;i<m/2;i++) printf("%c",c[i]);
        for (i=0;i<m/2;i++) printf("%c",c[i]);
        printf("\n");
    }
}