The given sentence states that the beavers have built a perfect dam across the stream. We need to rewrite this sentence in the passive voice.
To rewrite the sentence in the passive voice, we need to focus on the subject that receives the action (the dam) rather than the subject that performs the action (the beavers). We also need to change the verb form accordingly.
The correct answer is option B:
A perfect dam has been built by the beavers across the stream.
Here's the breakdown of the options:
- Option A:
A perfect dam had been built by the beavers across the stream.
- This is in the past perfect tense, indicating that the action of building the dam occurred before another past action or time. However, there is no indication of a specific past event in the original sentence, so this option is incorrect.
- Option B:
A perfect dam has been built by the beavers across the stream.
- This is in the present perfect tense, indicating an action that started in the past and has relevance to the present. This option correctly conveys the information in the original sentence.
- Option C:
A perfect dam have been built by the beavers across the stream.
- The verb "have" is not in agreement with the subject "a perfect dam." It should be "has" instead. Therefore, this option is grammatically incorrect.
- Option D:
A perfect dam was being built by the beavers across the stream.
- This is in the past continuous tense, indicating an ongoing action in the past. However, the original sentence does not imply an ongoing action, so this option is incorrect.
Therefore, the correct answer is option B:
A perfect dam has been built by the beavers across the stream.