单项选择题

现有:     
class TestA  { 
public void start()  {  System.out.println("TestA");  }     } 
public class TestB extends TestA  { 
public void start()  {  System.out.println("TestB");  }     public static void main (string[]  args)  (     ((TestA)new TestB()).start();     )     } 
运行结果是哪项?()     

A.  TeStA
B.  TeStB
C.编译失败
D.运行时抛出异常

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