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.

1. 

_ stt _ tt _ tts _

A. tsts
B. ttst
C. sstt
D. tsst

2. 

gfe _ ig _ eii _ fei _ gf_ ii

A. eifgi
B. figie
C. ifgie
D. ifige

3. 

_ a _ b _ abaa _ bab _ abb

A. aaabb
B. ababb
C. babab
D. babba

4. 

abca _ bcaab _ ca _ bbc _ a

A. ccaa
B. bbaa
C. abac
D. abba

5. 

_ tu _ rt _ s _ _ usrtu _

A. rtusru
B. rsutrr
C. rsurtr
D. rsurts