In alphabet series, some alphabets are missing which are given in that order as one of the alternatives below it. Choose the correct alternative.
abca _ bcaab _ ca _ bbc _ a
abca _ bcaab _ aa _ caa _ c
aab _ ab _ cabcca _ bcab _ c
_ cb _ ca _ bacb _ ca _ bac _ d
a _ bbc _ aab _ cca _ bbcc
a _ ca _ bc _ bcc _ bca