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)