#include <iostream>
#include <stdio.h>
#include <math.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <string>
#include <bitset>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <algorithm>
#include <sstream>
#include <stack>
#include <iomanip>
using namespace std;
#define pb push_back
#define mp make_pair
typedef pair<int,int> pii;
typedef long long ll;
typedef double ld;
typedef vector<int> vi;
#define fi first
#define se second
#define fe first
#define FO(x) {freopen(#x".in","r",stdin);freopen(#x".out","w",stdout);}
#define Edg int M=0,fst[SZ],vb[SZ],nxt[SZ];void ad_de(int a,int b){++M;nxt[M]=fst[a];fst[a]=M;vb[M]=b;}void adde(int a,int b){ad_de(a,b);ad_de(b,a);}
#define Edgc int M=0,fst[SZ],vb[SZ],nxt[SZ],vc[SZ];void ad_de(int a,int b,int c){++M;nxt[M]=fst[a];fst[a]=M;vb[M]=b;vc[M]=c;}void adde(int a,int b,int c){ad_de(a,b,c);ad_de(b,a,c);}
#define es(x,e) (int e=fst[x];e;e=nxt[e])
#define VIZ {printf("digraph G{\n"); for(int i=1;i<=n;i++) for es(i,e) printf("%d->%d;\n",i,vb[e]); puts("}");}
#ifdef LOCAL
#define TIMER cerr<<clock()<<"ms\n"
#else
#define TIMER
#endif
#define SZ 666666
typedef bitset<1001> bs;
int T,n,m;
char s[SZ];
int ab(int x)
{
    return (x>=0)?x:(-x);
}
typedef pair<char,int> pic;
pic ts[1003][1003];
void sol()
{
    memset(ts,127,sizeof(ts));
    pic inf=ts[0][0];
    ts[0][0]=pic(-1,-1);
    scanf("%d%d%s",&n,&m,s);
    int h=n/2;
    for(int i=h-1;i>=0;i--)
    {
        int p=h-i;
        int rp[3]={};
        for(int j='z';j>='a';j--)
        {
            int d=bool(j-s[i])+bool(j-s[h+i]);
            rp[d]=j;
        }
        for(int j=0;j<3;j++)
        {
            if(!rp[j]) continue;
            for(int k=0;k<=m;k++)
            {
                if(k<j) continue;
                if(ts[p-1][k-j]==inf) continue;
                ts[p][k]=min(ts[p][k],pic(rp[j],k-j));
            }
        }
    }
    if(ts[h][m]==inf)
    {
        puts("Impossible");
        return;
    }
    int cur=m;
    for(int i=h;i>=1;i--)
    {
        printf("%c",ts[i][cur].fi);
        cur=ts[i][cur].se;
    }
    cur=m;
    for(int i=h;i>=1;i--)
    {
        printf("%c",ts[i][cur].fi);
        cur=ts[i][cur].se;
    }
    puts("");
}
int main()
{
    scanf("%d",&T);
    while(T--) sol();
}