继承题

Given:
1. class One{
2.   public One foo(){return this;}
3. }
4. class Two extends One{
5.   public One foo(){return this;}  
6. }
7. class Three extends Two{
8.   //insert method here  
9. }
Which two methods, inserted individually, correctly complete the Three class? (Choose two.)
A. public void foo(){}
B. public int foo(){return 3;}
C. public Two foo(){return this;}
D. public One foo(){return this;}
E. public Object foo(){return this;}
答案：CD