/* *********************************
Author        :clogos
Created Time  :2016/09/24 20:03:53
File Name     :B.cpp
********************************** */

#include <bits/stdc++.h>
#define IOS ios::sync_with_stdio(0);cin.tie(0);
#define IN freopen("in", "r", stdin);
#define OUT freopen("out", "w", stdout);
using namespace std;
const int N = 1000 + 10;

char str[N], ans[N];

bool ok(int t_two, int t_one, int m, int len) {
    if((m < 0) || (m > 2 * t_two + t_one) || ((m & 1) && t_one == 0) || (m > len)) return false;
    else return true;
}

int main()
{
    int cases;    
    scanf("%d", &cases);
    while(cases--) {
        int n, dn, m;
        scanf("%d%d", &n, &m);
        m = n - m;
        dn = n / 2;
        scanf("%s", str);
        int t_two = 0, t_one = 0;
        for(int i = 0; i < dn; ++i) {
            if(str[i] == str[i+dn]) t_two++;
            else t_one++;
        } 
        if(!ok(t_two, t_one, m, n)) { printf("Impossible\n"); continue; }
        for(int i = 0; i < dn; ++i) {
            if(str[i] == str[i+dn]) {
                if(str[i] == 'a') {
                    if(ok(t_two-1, t_one, m-2, n-2*(i+1))) { ans[i] = ans[i+dn] = 'a'; m -= 2; }
                    else ans[i] = ans[i+dn] = 'b';
                }
                else {
                    if(ok(t_two-1, t_one, m, n-2*(i+1))) { ans[i] = ans[i+dn] = 'a'; }
                    else { ans[i] = ans[i+dn] = str[i]; m -= 2; }
                }
                t_two--;
            }
            else {
                if(str[i] == 'a' || str[i+dn] == 'a') {
                    if(ok(t_two, t_one-1, m-1, n-2*(i+1))) { ans[i] = ans[i+dn] = 'a'; m -= 1; }
                    else {
                        if(str[i] == 'b' || str[i+dn] == 'b') ans[i] = ans[i+dn] = 'c';
                        else ans[i] = ans[i+dn] = 'b';
                    }
                }
                else {
                    if(ok(t_two, t_one-1, m, n-2*(i+1))) { ans[i] = ans[i+dn] = 'a'; }
                    else { ans[i] = ans[i+dn] = min(str[i], str[i+dn]); m -= 1; }
                }
                t_one--;
            }
        }
        for(int i = 0; i < n; ++i) printf("%c", ans[i]);
        printf("\n");
    }
    return 0;
}