以下代码输出啥？为什么会这样输出？

{
    class Program
    {
        static void Main(string[] args)
        {
            A a = new B();
            a.Fun();
            Console.ReadKey();
        }
    }

    abstract class A
    {
        public A()
        {
            Console.WriteLine("A");
        }
        public virtual void Fun() {
            Console.WriteLine("A.Fun()");
        }
    }
    class B : A
    {
        
        public B()
        {
            Console.WriteLine("B");
        }
        public override void Fun()
        {
            Console.WriteLine("B.Fun()");
        }
    }
}
{
    class Program
    {
        static vo