Q:

The beavers have built a perfect dam across the stream  

• 1
  A perfect dam had been built by the beavers across the stream.
• 2
  A perfect dam has been built by the beavers across the stream.
• 3
  A perfect dam have been built by the beavers across the stream.
• 4
  A perfect dam was being built by the beavers across the stream