The below sentence has been given in Active/Passive voice. From the given alternatives, choose the one which best expresses the given sentence in Passive/Active voice.They have built a perfect dam across the river.

• A. Across the river a perfect dam was built.
• B. A perfect dam has been built by them across the river.
• C. A perfect dam should have been built by them.
• D. Across the river was a perfect dam.