A cafe on the ground floor of our office building has a screen which cycles through photos and short videos, weather forecasts, book, tv, movie and podcast reviews, and, relevantly, trivia questions. Yesterday, one of these was “When [or maybe What year] did they open the London Underground?”. My guess (1861) was reassuringly close to the actual answer (1863). (I have browsed through Wikipedia’s articles on various rail networks, but have never memorised relevant dates.) But I couldn’t help thinking about why they chose to ask the question like that.
There’s nothing wrong with the question as it is; active voice is the default within the English verb system. But it throws too much emphasis onto they, when we either don’t know or don’t care who they were. Who opened the London Underground? Some representatives of royalty, government, commerce and industry, probably. Passive voice is useful in situations like this.
You may have noticed that I wrote “But I couldn’t help thinking about why they chose to ask the question like that”. Same question: who are they? We don’t know individually, but we know there was a person (they!) or people who chose to ask the question like that. Besides, that clause doesn’t have a direct object to become the subject of the passive voice alternative. Compare “They chose that question to demonstrate active voice” > “That question was chosen to demonstrate passive voice” or “Why did they chose that question to demonstrate active voice?” > “Why was that question chosen to demonstrate passive voice?”.