Which one of the following is an example of a set S such that the language S* has the same number of 6-letter words and 3-letter words?
1) {a, aa, aaa}
2) {a, aa, aaa, aaaa}
3) {a, aa, aaa, aaaa, aaaaa}
4) {a, aa, aaa, aaaa, aaaaa, aaaaaa}