Choose the answer which correctly completes the sentence

Choose the answer which correctly completes the sentence.

The boredom that often results from performing the same task all day is a major problem for some workers on assembly lines, … ?

1. 
   Isn`t it B) doesn`t it C) aren`t they D) will you
   

19. 
    Choose the answer which correctly completes the sentence.
    

… kind of wood ha its own characteristics which set it apart from … types and determine its suitability for certain uses.

1. 
   All/several B) Every/either C) Each/other D) None/both
   

20. 
    Choose the answer which correctly completes the sentence.
    

The Lusitania passenger ship was sunk without warning by a German submarine in the Atlantic in 1915, … 1,200 people aboard.

1. 
   Was killed B) to be killed C) having been killed D) killing
   

21.