Who is that? vs Who is it?

[MangeshD]

Which one is grammatically correct;
Who is that? / Who is it?

[Kitosdad]

Both are correct.

Asking a friend, "Who is that?" in reference to a third person.

When you are at home and someone knocks on your door you may ask, " Who is it?" if you don't wish to open the door.
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