有如下这样一段代码：
using System;
using System.Collections.Generic;
using System.Text;

namespace methoddemo
{
    abstract class A
    {
        int Method1()
        {
            return 0;
        }
        int Method1(int i)
        {
            return 1;
        }
        int Method1(double b)
        {
            return 2;
        }
        public static void Method2()
        { }
        public void F()
        {
            Console.WriteLine("A.F");
        }
        public virtual void G()
        {
            Console.WriteLine("A.G");
        }
        public abstract void Method3();
        
    }
    class B : A
    {
        new public void F()
        {
            Console.WriteLine("B.F");
        }
        public override void G()
        {
            Console.WriteLine("B.G");
        }
        public override void Method3()
        {
            Console.WriteLine("implement abstract method3");  
        }
    }
    class Test
    {
        static void Main(string[] args)
        {
            B b = new B();
            A a = b;
            a.F();
            b.F();
            a.G();
            b.G();
        }
    }
}

大家觉得程序执行语句a.F();后的输出会是什么呢？因为A是抽象类，那么对于其中语句A a = b;是怎么理解的啊？谢谢大家……