Select the combination of letters that when sequentially placed in the blanks of the given letter series will complete the series. ab_ _ _cba_ _ _dd_ _ _abcd_ _baa_ _ _ _cba

acbdecdeaacecdd cddabccbadcbcdd abeaacdecdeecdd abbaacdeecdecdd

cddabccbadcbcdd

a b c d / d c b a / a b c d / d c b a / a b c d / d c b a / a b c d / d c b a c d d a b c c b a d c b c d d