简单一个题,该如何解决

简单一个题
2.   1)   class   Super{  
2)   public   float   getNum(){return   3.0f;}  
3)   }  
4)  
5)   public   class   Sub   extends   Super{  
6)  
7)   }  
which   method,   placed   at   line   6,   will   cause   a   compiler   error?  
A.   public   float   getNum(){return   4.0f;}  
B.   public   void   getNum(){}  
C.   public   void   getNum(double   d){}  
D.   public   double   getNum(float   d){return   4.0d;}  
Answer:B  

为什么选它啊

------解决方案--------------------
A. public float getNum(){return 4.0f;} //重写
B. public void getNum(){} //错误 方法名与父类同 ,而且参数相同,但返回类型不同是不行的
C. public void getNum(double d){}//重载
D. public double getNum(float d){return 4.0d;} ///重载