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.

16. 

ab _ aa _ bbb _ aaa _ bbba

A. abba
B. baab
C. aaab
D. abab

17. 

_ nmmn _ mmnn _ mnnm _

A. nmmn
B. mnnm
C. nnmm
D. nmnm

18. 

a _ n _ b _ _ ncb _ _ ncb

A. abbbec
B. abebeb
C. bacbab
D. bcabab

19. 

a _ ba _ b _ b _ a _ b

A. abaab
B. abbab
C. aabba
D. bbabb

20. 

cccbb _ aa _ cc _ bbbaa _ c

A. aebe
B. baca
C. baba
D. acba