In each of the following letter series, some of the letters are missing which are given in that order as one of the alternatives below it. Choose the correct alternative.

6. 

_ _aba _ _ ba _ ab

A. abbba
B. abbab
C. baabb
D. bbaba

7. 

c _ bbb _ _ abbbb _ abbb _

A. aabcb
B. abccb
C. abacb
D. baebb

8. 

_ bbca _ bcca _ ac _ a _ cb

A. abeba
B. acbab
C. bacab
D. bcaajb

9. 

a _ bbc _ aab _ cca _ bbcc

A. bacb
B. acba
C. abba
D. caba

10. 

ac _ cab _ baca _ aba _ acac

A. aacb
B. aebe
C. babb
D. bcbb