In alphabet series, some alphabets are missing which are given in that order as one of the alternatives below it. Choose the correct alternative.
c _ baa _ aca _ cacab _ acac _ bca
abb _ baa _ a _ bab _ aba
_ baa _ aab _ a _ a
aba _ baca _ ba _ bacaabac _ aca
ac _ cab _ baca _ aba _ acac
_ bbca _ bcca _ ac _ a _ cb