Challenge 1: Correct virtual/override Dynamic Dispatch — Possible Solution
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Program.cs:

class Shape
{
    public virtual double Area() => 0;
}

class Circle : Shape
{
    public double Radius;
    public Circle(double radius) { Radius = radius; }
    public override double Area() => Math.PI * Radius * Radius;
}

Shape s = new Circle(3);
Console.WriteLine(s.Area());

Output:

    28.274333882308138

Explanation:
Shape.Area() is marked virtual, and Circle.Area() is marked override
-- both keywords present, exactly as the chapter requires for dynamic
dispatch. Even though `s` is declared as type Shape, calling s.Area()
runs Circle's overridden version, because the CLR dispatches based on
the object's real runtime type (a Circle) whenever virtual/override
are both present, the same outcome java1-5's Java would produce
automatically without either keyword.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates dynamic dispatch working correctly through a base-
typed variable, using both required keywords exactly as the chapter's
own Animal/Dog example shows, confirming the polymorphic behavior
Java gives by default still exists in C# -- it just has to be asked
for explicitly.