Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series. a b b _ b_ _ b a b _ _ b a b b b b_ _

b a b b a a b a b b a a b b a b b a b a b a b b a a a b

a b b a b a b

a b b a b b b b a b / a b b a b b b b a b a b b a b a b