"He likes the zoo."
Translation:Anh ấy thích sở thú.
The information is in the Tips & Notes Section in Animals 1. Usually, when people refer to a specific noun, such as a specific cat or dog, then you would use a classifier. When you are referring a quantity, then you also use a classifier. But when you are talking about all cats or dogs in general, then it is not required to use a classifier.