()在C#程序中,有如下代码,则下面选项中说法正确是（）public class A{}pub

()在C#程序中,有如下代码,则下面选项中说法正确是（）public class A{}public class B:A{}A a=new A();B b=new B();A.表达式b as A的值为nullB.表达式a as B的值为nullC.表达式b is A的值为trueD.表达式a is B的值为true