C++类组成，为什么不调用构造函数

我有以下代码，我不明白，为什么在创建对象 b 时，类 A 的构造函数不调用......我会很感激你的一点帮助。

#include <iostream>
using namespace std;
class MyClass
{
    int x;
public:
    MyClass(int y);                           
    MyClass(MyClass &my) 
    {
        x = my.x;
        cout << "My class created by copy" << endl;
    }
};
MyClass::MyClass(int y)
{
    x = y;
    cout << "My class created" << endl;
}
class A {
    MyClass k;
    public:
        A(MyClass &my) : k(my) {
            cout << "A created" << endl;
        }
};
class B {
    A data;
    public:
        B(A& aa) : data(aa)
        {
            cout << "B created" << endl;
        }
};
int main()
{
    MyClass obj(100);
    A a(obj);
    B b(a);                     
    return 0;
}

执行：

My class created
My class created by copy
A created
My Class created by copy
B created