lass ClassA {} class ClassB extends ClassA {) class ClassC extends ClassA {) 以及: ClassA p0=new ClassA(); ClassB pl=new ClassB(); ClassC p2=new ClassC(); ClassA p3=new ClassB(); ClassA p4=new ClassC(); 下列哪些是正确的? A. p0=pl; B. p1 = (ClassB)p3 ; C. p2 = (Classc)p4; D. p1 =p2;