I don't mean to offend the first respondent, but her answer was incorrect.
I am 1,000,000,000% certain that this is the correct answer.
C. The effects of cyanide are well known.
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Passive voice indicates that the subject of the sentence (effects of cyanide) is not the source of the action.
We can turn this example sentence around to say: "[Most people] know well the effects of cyanide."
We cannot do the same with the other sentences. All their subjects are the source of the action.