父类作为函数参数，析构函数的调用问题

#include
using namespace std;

class A
{
public:
A(){cout<<"A constructor\n";}
~A(){cout<<"A destroy\n";}

};
class B : public A
{
public:
B(){cout<<"B constructor\n";}
~B(){cout<<"B destroy\n";}

};

void func(A x){
A y = x;
}

void main()
{
B z;
func(z);
}
为什么func(z)中调用了 3 次A的析构函数？