My classmate wrote a text about zoos in general, and the word "zoos" in plural form is mentioned many times in it. Later in the text he uses singular form "zoo" two times with indefinite articles, and both times he still refers to an abstract zoo, not to specific one.
The place where animals can obtain shelter is a zoo.
You can take the kids to a zoo.
I'm having trouble in figuring out whether it is correct or not. I think that he should have used a definite article the second time, because he already mentioned a singular abstract zoo, but he argues that he refers to "two different singular abstract zoos".
So which article should be used in a second sentence?