static void Main(string[] args)
        {
            Rabbit rabbit = new Rabbit();

            rabbit.Name = "Cute01";
            rabbit.Age  = -10;
            Console.WriteLine(rabbit.Age);

            int x = default; //0
        }
        static void Main(string[] args)
        {
            var rabbit = new Rabbit();

            rabbit.Name = "Cute01";
            // rabbit._bloodType.. INVALID
            rabbit.Age = -10;
            Console.WriteLine(rabbit.Age);

            int x = default; // 0
        }