I couldn’t find these explanation of exactly how the rules of articles apply in questions like this:
Why do people go to zoos?
Why do people go to zoo?
Why do people go to the zoo?
It’s not a question about a specific zoo, but about zoos in general.
The first variant sounds correct, but I’m not so sure about the second and third.
Are there scenarios in which both are acceptable, such as spoken English versus formal writing?