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- Кто был в Барселоне?

- Все были.

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“Кто” grammatically is “он” (singular masculine). Therefore questions with “кто” require a verb in singular masculine (“он”-form): был, REGARDLESS the answer.

The form of a verb in an answer shouldn’t necessarily be the same as its form in a question.

“Все” grammatically is “они” (plural), so, it requires a verb in the plural form (“они”-form): были.

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