Although this question is one year old, I'll leave an answer for the sake of new students: The verb "to be" is implied in это, that's why this kind of sentence always use это in Russian, otherwise the sentence would end up with no verb.
Wondering the same.. because already the translation says it, "whose are THESE tables", not "these ARE tables, whose?". Even the word это is located in the middle so it doesn't make any sense.
эти столы = these tables; also These tables ans not another tables это столы= There are tables (and not chairs or something Else)
This is exactly why I feel the question should be with эти столы, as the subject are these specific tables whose owners aren't known
It is possible. Pretend you're going to say чай, for example, but stop before reaching а.
If I was in school and in the hallway are a lot of tables, would the teacher say that phrase, too? Can you say "чьи", too, although they don't really have an owner, but it's more important to which classroom they belong.?