关于c#的问题，帮忙补全一下代码！

using System;

namespace ConsoleApplication2
{
    class Graph
    {
        public virtual void Draw()
        {
            Console.WriteLine("This is a graph!");
        }
    ｝
    class Rectangle:Graph
    {
            public override void Draw()
            {
                Console.WriteLine("This is a rectangle!");
            }
    }
    class Circle:Graph
    {
            public override void Draw()
            {
                Console.WriteLine("This is a circle!");
            }
     }
    }
    class Program
    {
        static void Main(string[] args)
        {
            //声明基类对象Graph g，并构造为Rectangle类型
            //调用g的Draw方法
            //再将g构造为Circle类型
            //调用g的Draw方法  
        }
    }
}

class Program
{
static void Main(string[] args)
{
//声明基类对象Graph g，并构造为Rectangle类型
//调用g的Draw方法
//再将g构造为Circle类型
//调用g的Draw方法
}
}

 
using System;
namespace ConsoleApplication2
{
    class Graph
    { 
         public Graph()
{
}
        public virtual void Draw()
        {
            Console.WriteLine("This is a graph!");
        }      
    }
 class Rectangle:Graph
        {
            public override void Draw()
            {
                Console.WriteLine("This is a rectangle!");
            }
        }
        class Circle:Graph
        {
            public override void Draw()
            {
                Console.WriteLine("This is a circle!");
            }
        }
    class Program
    {
        static void Main(string[] args)
        {
            Graph g = new Graph();
            g.Draw();
          Rectangle rect = new Rectangle();
rect.Draw();
        }
    }
}

using System;

namespace ConsoleApplication2
{
    class Graph
    {
        public virtual void Draw()
        {
            Console.WriteLine("This is a graph!");
        }
    }
    class Rectangle : Graph
    {
        public override void Draw()
        {
            Console.WriteLine("This is a rectangle!");
        }
    }
    class Circle : Graph
    {
        public override void Draw()
        {
            Console.WriteLine("This is a circle!");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            //声明基类对象Graph g，并构造为Rectangle类型
            //调用g的Draw方法
            //再将g构造为Circle类型
            //调用g的Draw方法  
            Graph g = new Rectangle();
            g.Draw();
            g = new Circle();
            g.Draw();
        }
    } 
}