Select the combination of letters that when sequentially placed in the blanks of the given letter series will complete the series. ac_ b_ ac _ _aa _d _ a _ cd_ _

dadbcbaba bcdcdaba cbdcabada acbdcdabc

dadbcbaba

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