Select the combination of letters that when sequentially placed in the blanks of the given letter series will complete the series. a _ _ gb _ _ acegb _ _ aceg _ _ _ |
dfdfbcedf cedfdfbdf fdfbdfced cefbdfdfd |
cedfdfbdf |
a c e g b d f / a c e g b d f / a c e g b d f c e d f d f b d f |