若以下代码出问题,请指出主要是哪一行引起的。①sealed class A{Publicvoid MethodA(){Console.WriteLine("this is A class");}}②class B:A{private string _name;private int _age;public B(string name, int age){_name = name;_age = age;}public void MethodB(){Console.WriteLine("this is B class");}}class Program{static void Main(string[] args){③B b = new B("aaa",20);b.MethodA();④b.MethodB();}}