#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<vector>
#include<cmath>
using namespace std;
char c[1005];
char d[1005];
bool dp[505][1005];
int main()
{
    int t;
    cin>>t;
    while(t--){
        int n,m;
        scanf("%d%d",&n,&m);
        scanf("%s",c);
        for(int i=n;i>=1;i--) c[i]=c[i-1];
        for(int i=0;i<=m;i++) dp[n/2+1][i]=0;
        dp[n/2+1][m]=1;
        for(int i=n/2;i>=1;i--){
            for(int j=0;j<=m;j++){
                dp[i][j]=0;
                if(c[i]==c[n/2+i]){
                    if(j+2<=m&&dp[i+1][j+2]) dp[i][j]=1;
                    if(dp[i+1][j]) dp[i][j]=1;
                }
                else{
                    if(j+1<=m&&dp[i+1][j+1]) dp[i][j]=1;
                    if(j+2<=m&&dp[i+1][j+2]) dp[i][j]=1;
                }
            }
        }
        if(dp[1][0]==0) printf("Impossible\n");
        else{
            int no=0;
            for(int i=1;i<=n/2;i++){
                if(c[i]==c[i+n/2]){
                    char ma='z';
                    int f=0;
                    if(no+2<=m&&dp[i+1][no+2]){
                        if(c[i]=='a'){
                            ma='b';
                        }
                        else{
                            ma='a';
                        }
                        f=2;
                    }
                    if(dp[i+1][no]){
                        if(ma>c[i]){
                            f=0;
                            ma=c[i];
                        }
                    }
                    d[i]=d[i+n/2]=ma;
                    no+=f;
                }
                else{
                    char ma='z';
                    int f=0;
                    if(no+2<=m&&dp[i+1][no+2]){
                        char x=min(c[i],c[i+n/2]);
                        char y=max(c[i],c[i+n/2]);
                        if(x!='a') ma='a';
                        else{
                            if(y=='b') ma='c';
                            else ma='b';
                        }
                        f=2;
                    }
                    if(no+1<=m&&dp[i+1][no+1]){
                        if(ma>min(c[i],c[n/2+i])){
                            ma=min(c[i],c[n/2+i]);
                            f=1;
                        }
                    }
                    d[i]=d[i+n/2]=ma;
                    no+=f;
                }
            }
            for(int i=1;i<=n;i++) printf("%c",d[i]);
            printf("\n");
        }
    }
}