Which class/set of classes can illustrate polymorphism in the following C++ code?

abstract class student
{
   public : int marks;
   calc_grade();
}
class topper:public student
{
    public : calc_grade()
    { 
        return 10; 
    }
};
class average:public student
{ 
     public : calc_grade()
     {
         return 20; 
     }
};
class failed{ int marks; };