public class Plane {   public virtual double TopSpeed() { return 300.0D;}   public static bool operator>(Plane one, Plane two) {       return one.TopSpeed() > two.TopSpeed();   }   public static bool operator<(Plane one, Plane two) {       return one.TopSpeed() < two.TopSpeed();   }  }  class Jet : Plane {   public override double TopSpeed() { return 900.0D; }   public override string ToString() { return "I'm a Jet"; }  }   class Airport {   [STAThread]   static void Main(string[] args) {     Plane plane = new Jet();     Console.WriteLine("plane's top speed: {0}",plane.TopSpeed());     Jet jet = new Jet();     Console.WriteLine("jet's top speed: {0}",jet.TopSpeed());     Console.WriteLine("Plane > Jet = {0}", plane > jet);     Console.ReadLine();   }  }