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单选题
类Teacher和Student是类Person的子类;      Person p;      Teacher t;      Student s;   //p, t and s are all non-null.   if(t instanceof Person) { s = (Student)t; }     最后一句语句的结果是:()
A

 将构造一个Student对象;

B

 表达式是合法的;

C

 表达式是错误的;

D

 编译时正确,但运行时错误。


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