C中的虚函数

#include<iostream.h>

class A
{
    public:
    virtual void func()
    {
         cout<<"In A"<<endl;
    }
};

class B:public A
{
    public:
    void func()
    {
        cout<<"In B"<<endl;
    }
};

class C:public B
{
    public:
    void func()
    { 
        cout<<"In C"<<endl;
    }
};  

int main()
{
    B *ptr=new c;
    ptr->func();
}

该语句应该调用B :: func().但是,调用函数C :: func().请详细说明一下.一旦在’A类’中删除了虚拟关键字,就不会再发生这种情况了.