abstract class BaseClass{ public virtual void MethodA() { Console.WriteLine("BaseClass"); } public virtual void MethodB(){} } class Class1:BaseClass{ public void MethodA() { Console.WriteLine("Class1");} public virtual void MethodB(){} } class Class2:Class1{ public void MethodB(){} } class MainClass{ public static void Main(string[] args){ Class2 c = new Class2(); c.MethodA(); } }请问,此程序输出的结果是(选一项) A. BaseClass B. BaseClass Class1 C. Class1 D. Class1 BaseClass