Select the combination of letters that when sequentially placed in the blanks of the given letter series will complete the series. abx _ dab _ _ d _ b _ xdab _ x _ |
dxadadd xxxaxxd xxcbxad axdaxxd |
xxxaxxd |
a b x x d / a b x x d / a b x x d/ a b x x d x x x a x x d |