Computer theory: Kleene closure

[annitaz]

Which one of the following is an example of two sets S and T of strings such that S ⊄ T and T ⊄ S, but S* = T*?

 

1. S = {a, b, abb, bba} and T = {a, b, bba}

2. S = {a, aaaa} and T = {a, aaa, aaaa}

3. S = {a, ab, aab} and T = {a, ab, abab}

4. S = {a, b, aaa} and T = {a, bbb}

5. S = {bb, bbbb} and T = {b}

 

I have excluded answers 1,2,5 but I am stuck with 3 and 4. When is S* equal to T*?