Why not use :base to pass all the parameter?

Question

This exercise is part of HackerRank 30 days of Code - Day 13 Abstract Class C#

I am trying to understand the reason why we have to define an int price = 0 in this case when we did not have to do the same for the title and author for example. Why we can't just pass the parameter using :base as we did with title and author?

Here is the code in C#:

class MyBook : Book {
   
    public int price = 0;
    
    public MyBook(String title, String author, int price) 
           : base(title, author)
      {
        this.price = price;  
      }
    
    public override void display()
        {
            Console.WriteLine($"Title: {title}");
            Console.WriteLine($"Author: {author}");
            Console.WriteLine($"Price: {price}");
        }
    
}

Thank you for your help!