Which three are valid?()<br /> <img src="https://nimg.ppkao.com/2018-08/zhoulian/2018080111555918424.jpg?sign=295f279cabfff46e3e85be54bbb0d2df&t=62d1ded9" />
A、p0=p1;
B、p1=p2,
C、p2=p4;
D、p2=(ClassC)p1;
E、p1=(ClassB)p3;
F、p2=(ClassC)p4;