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多项选择题

class ClassA {} 
class ClassB extends ClassA {} 
class ClassC extends ClassA {} 
and: 
ClassA p0 = new ClassA(); 
ClassB p1 = new ClassB(); 
ClassC p2 = new ClassC(); 
ClassA p3 = new ClassB(); 
ClassA p4 = new ClassC(); 
Which three are valid?()

A. p0 = p1;
B. p1 =p2;
C. p2 = p4;
D. p2 = (ClassC)p1;
E. p1 = (ClassB)p3;
F. p2 = (ClassC)p4;

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