The answer to your question would be that when a writer does not know who performed the action in a sentence, the writer can still write a grammatically correct sentence by using the passive voice. That is, the correct option would be B.
You can change the word order of the active sentences with a direct object for the subject to be no longer active, but acted upon by the verb or passive. Take A & B as examples:
a) Mary fed the dog (active sentence)
b) The dog was fed (passive sentence)