They were building it
Answers
It was being built by them. This would be the answer if you want it in passive voice.
Given sentence:
They were building it.
Sentence after adding question tag:
They were building it, weren't they?
Question tag - weren't they?
Sentence in Passive voice:
It was being built by them.
OR It was being built. ("by them" can be ignored)
Additional information:
Active voice - A type sentence where in the subject of the sentence precede the object of the sentence.
Passive voice - A type of sentence wherein the object of the sentence precede the subject of the sentence.
Question tag may be simply and compactly defined as a single phrase or group of words that is added at the end of a statement or sentence in order to and converts it to a question.
The British English call it "Question Tag" whereas on the other hand American English call it "Tag Question" or "Tail Question".
Question tags are generally common in familiar conversation and not in formal writing or conversation.