Which two methods, inserted individually, correctly complete the Three class?

Given:

10. class One {
11. public One foo() { return this; }
12. }
13. class Two extends One {
14. public One foo() { return this; }
15. }
16. class Three extends Two {
17. // insert method here
18. }

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; }