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.

21. 

bca _ b _ aabc __ a __ caa

A. acab
B. bcbb
C. cbab
D. ccab

22. 

ab __ d __ aaba __ na _ badna _ b

A. andaa
B. babda
C. badna
D. dbanb

23. 

m _ nm _ n _ an _ a _ ma _

A. aamnan
B. ammanm
C. aammnn
D. amammn

24. 

_bcc _ ac _ aabb _ ab _ cc

A. aabca
B. abaca
C. bacab
D. bcaca

25. 

_ op _ mo _ n _ _ pnmop _.

A. mnpmon
B. mpnmop
C. mnompn
D. mnpomn