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I would like to know why 'Who is there?' is correct and why we can't use 'Who are there?' even if we know there is more than one person.

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Who is used as a singular or a plural pronoun. If you use the who as an interrogative pronoun for a person, it takes a singular verb. If you use it for more than a person, it generally takes a singular verb, though the use of a plural verb is not ungrammatical. So both the following sentences are correct.

Who is there.

Who are there.

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