有如下代码,请问哪些是正确的 ?() class ClassA{} class ClassB extends ClassA{} class ClassC extends ClassA{} 以及 ClassA p0 = new ClassA(); ClassB p 1 = new ClassB(); ClassC p2 = new ClassC(); ClassA p3 = new ClassB(); ClassA p4 = new ClassC(); A. p0 = p1; B. p1 = p2; C. p1 = (ClassB)p3; D. p2 = (ClassC)p4;