Question

In the case of difficulty you should refer -----dictionary -preposition

Answer 1

in the case of difficulty you should refer TO dictionary.

Answer 2

Prepositions are used to express the relationship between two or more places, persons or time or express a relationship in time or space. For example:

The Last Time I met him was opposite the cinema. - place

Reena was walking down the street. - place

I love eating ice-creams during summers. - time

Therefore, in completeing teh above sentence with preposition it would be written as: "In the case of difficulty you should refer to dictionary."

Herein, preposition 'to' is used in phrasal verb.