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- Кто был вчера на конференции?

- Там были ребята из Google и Microsoft.

- А что там было?

- Была презентация нового проекта Google.

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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. Thus, in this dialog the answer is “ребята” (“они”), and the verb takes “они”-form (the plural form): были.

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“Что” grammatically is “оно” (singular neuter). Therefore questions with “что” require a verb in singular neuter (“оно”-form): было, REGARDLESS the answer.

For example, here the answer is “презентация” (“она”), and we use the verb in “она”-form (singular feminine): была.

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