A letter series is given below in which some letter are missing. Select the  option that gives the letters that can fill these blanks in that order.
a_ca_bab_ad_abc_db

Choose the correct option from the following options.
8 : 9 ∷ 64 : ?

In each of the following questions, which one of the four interchanges in signs and numbers would make the given equation correct?
( 3÷4 ) + 2=2 

X, Q, K, F, ?  

Six friends A, B, C, D, E and F are standing in a row facing north. B stands exactly between F and D. E stands exactly between A and C. A is not a neighbor of D and F. C is not a neighbor of D but is to the immediate right of F. C stands between which of the following.

Virology : Virus :: Semantics : ? 

In the following question, select the related word from the given alternatives.
Encouragement: Victory :: Frustration: ?

DIRECTIONS: Study the following information and answer the given questions:
A person goes 9 m straight. He turns 120^o clockwise and goes 41 m. Then, he turns 210^O anticlockwise and goes 44 m. Finally, he turns 150^O clockwise and goes 5 m.
How far is he from his starting point?