如果类 Student 是类 Person 的子类,类 Undergraduate 是类 Student 的子类,请指出以下代码行中哪些是不合法的。 Person p1=new Student(); Person p2=new Undergraduate(); Student s1=new Person(); Student s2=new Undergraduate(); Undergraduate ug1=new Person(); Undergraduate ug2=new Student(); Object ob=new Student();