A sentence has been given in Active/Passive voice. Out of the four alternatives suggested below, select the one which best expresses the same sentence in Passive/Active voice. 
 

The agent had disclosed the unknown before it was evening. 

The unknown had been disclosed by the agent before it was evening.

The unknown had been disclosed by the agent before it was evening.

Explanation:

• The unknown was disclosed by the agent before it was evening.: This passive construction reverses the subject and object but doesn't accurately capture the original tense and sequence of events.
• The unknown had disclosed by the agent before it had been evening.: This construction is not grammatically correct, as it should be "had been disclosed."
• The unknown had been disclosed by the agent before it was evening.: This passive construction correctly maintains the original tense and sequence of events while forming the passive voice.
• The unknown was disclosed by the agent before it had been evening.: This passive construction is not accurate in terms of tense and sequence of events. The original sentence suggests that the agent disclosed the unknown before a specific time (evening), not before a period (had been evening).