为什么下面的程序要a b c的地址赋给指针p p1 p2?

#include<iostream>
using namespace std;
class A
{
public:
virtual void print(){cout<<"a"<<endl;}

};
class B:public A
{public:
void print(){cout<<"b"<<endl;}
};
class C:public A
{public:
void print(){cout<<"c"<<endl;}
};
int main()
{
A a;
B b;
C c;
A *p=&a;
A *p1=&b;
A *p2=&c;
p->print();
p1->print();
p2->print();

return 0;
}