/*头文件模板*/

#include <map>
#include <set>
#include <cmath>
#include <ctime>
#include <queue>
#include <stack>
#include <vector>
#include <cctype>
#include <cstdio>
#include <string>
#include <cstring>
#include <sstream>
#include <cstdlib>
#include <iomanip>
#include <typeinfo>
#include <iostream>
#include <algorithm>
#include <functional>

using namespace std;

#define pb push_back
#define mp make_pair
#define mem(a, x) memset(a, x, sizeof(a))
#define copy(a, b) memcpy(a, b, sizeof(a))
#define lson rt << 1, l, mid
#define rson rt << 1|1, mid + 1, r
#define FIN freopen("input.txt", "r", stdin)
#define FOUT freopen("output.txt", "w", stdout)

typedef long long LL;
typedef pair<int, int > PII;
typedef pair<int, string> PIS;
typedef pair<LL, LL>PLL;
typedef unsigned long long uLL;

template<typename T>
void print (T* p, T* q, string Gap = " ", bool flag = false) {
    int d = p < q ? 1 : -1;
    while (p != q) {
        if (flag) cout << Gap[0] << *p << Gap[1];
        else cout << *p;
        p += d;
        if (p != q && !flag) cout << Gap;
    }
    cout << endl;
}

template<typename T>
void print (const T &a, string bes = "") {
    int len = bes.length();
    if (len >= 2) cout << bes[0] << a << bes[1] << endl;
    else cout << a << endl;
}

template<typename T>
void debug (T* p, T* q, string Gap = " ", bool flag = false) {
#ifndef ONLINE_JUDGE
    int d = p < q ? 1 : -1;
    cout << "Debug out : ";
    while (p != q) {
        if (flag) cout << Gap[0] << *p << Gap[1];
        else cout << *p;
        p += d;
        if (p != q && !flag) cout << Gap;
    }
    cout << endl;
#endif
}

template<typename T>
void debug (const T &a, string bes = "") {
#ifndef ONLINE_JUDGE
    int len = bes.length();
    cout << "Debug out : ";
    if (len >= 2) cout << bes[0] << a << bes[1] << endl;
    else cout << a << endl;
#endif
}

void IO_Init() {
    ios::sync_with_stdio (false);
}

LL LLabs (const LL a) {
    return a >= 0 ? a : -a;
}

const double PI = 3.1415926535898;
const double eps = 1e-10;
const int MAXM = 1e5 + 5;
const int MAXN = 1e3 + 5;
const int INF = 0x3f3f3f3f;

/*头文件模板*/

int T;
int n, m;
char S[MAXN], SF[MAXN];
int SMa[MAXN], SMi[MAXN];
char J[MAXN];
int main() {
#ifndef ONLINE_JUDGE
    FIN;
    //FOUT;
#endif
    IO_Init();

    scanf("%d", &T);
    while(T --) {
        scanf("%d%d%s", &n, &m, S);
        int len = strlen(S);
        if(len & 1) {
            printf("Impossible\n");
            continue;
        }
        SMa[len / 2] = 0;
        SMi[len / 2] = 0;
        for(int i = len / 2 - 1; i >= 0; i --) {
            if(S[i] != S[i + len / 2]) {
                SMa[i] = SMa[i + 1] + 2;
                SMi[i] = SMi[i + 1] + 1;
            } else {
                SMa[i] = SMa[i + 1] + 2;
                SMi[i] = SMi[i + 1];
            }
        }
        bool flag = true;
        if(m > SMa[0] || m < SMi[0]) {
            flag = false;
        }

        int l = 0;
        while(l < len / 2 && flag) {
            int k1 = m - SMa[l + 1];
            int k2 = m - SMi[l + 1];
            k2 = max(0, k2);
            k1 = max(0, k1);
            char a = 'a';
            while(true) {
                if(a > 'z') {
                    flag = false;
                    break;
                }
                int bt = (a != S[l] ? 1 : 0) + (a != S[l + len / 2] ? 1 : 0);
                if(bt >= k1 && bt <= k2) {
                    J[l ++] = a;
                    m -= bt;
                    break;
                }
                a ++;
            }
        }
        if(!flag) {
            printf("Impossible\n");
            continue;
        }
        for(int i = 0; i < l; i ++) {
            printf("%c", J[i]);
        }
        for(int i = 0; i < l; i ++) {
            printf("%c", J[i]);
        }
        printf("\n");
    }
    return 0;
}