Passive voice is generally used when the agent (the noun doing the action) is unimportant or irrelevant or unknown. ' -- we don't know who killed him, but the important thing is that he died, not which enemy soldier did it. Passive is often used in scientific and similar papers, where we wish to describe the activity, but it is unimportant who does it: 'The rat is placed into a V7a maze and released.
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