4. In the passive voice, the agent is very often not mentioned. When it is mentioned, it is introduced by

the preposition ... at the end of the sentence a. ( ) in In
b. ( ) on
c.( ) at
( ) by
a.( )a, b, c, d.
b.( )b, b, d, c.
c.() b, b, c,d.
d.()d, c,a, b. d.

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