What is the output?

  class X
  {
      Y b = new Y();
      public X()
      {
          Console.Write("X");
      }
  }
  class Y
  {
      public Y()
      {
          Console.Write("Y");
      }
  }
  class Z : X
  {
      Y y = new Y();
      public Z()
      {
          Console.Write("Z");
      }
  }
  class Program
  {
      static void Main(string[] args)
      {
          new Z();
      }
  }