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- Кто это?

- Это мой брат.

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The question “Кто это?” is very wide and doesn’t require any special information about the person. So, “Кто это? – Это мой брат (Who is that? – That is my brother)” is a possible conversation. The question “Кто он?” doesn’t work for “Это мой брат” because this question normally requires  information about person’s profession or occupation. The variant “Антон” doesn’t work to complete the question because when we use names after “кто” we add a “blank” word “такой/ такая/ такие”: “Кто такой Антон?”

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