In the following questions, the sentences have 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.

Please do not use chat terms. Example: avoid using "grt" instead of "great".