一道C++的题目，费解，求教！

#include <iostream>
using namespace std;
class A
{
public:
A()
{printf("A constructed.\n");}
virtual ~A()
{printf("A deconstructed.\n");}
virtual void Fn()
{printf("A fn called.\n");}
};
class B:public A
{
public:
B()
{printf("B constructed.\n");}
virtual ~B()
{printf("B deconstructed.\n");}
virtual void Fn()
{printf("B fn called.\n");}
};

class C:public B
{
public:
C()
{printf("C constructed.\n");}
virtual ~C()
{printf("C deconstructed.\n");}
virtual void Fn()
{printf("C fn called.\n");}
};

int main(int argc,char* grgv[])
{
A *pA=new B;
if(pA!=NULL)
pA->Fn();
B *pB=static_cast<B*>(pA);

if(pB!=NULL)
pB->Fn();