Rule 4 says that,"when a question is asked with a verb of performance(main verb), we reply with a subject pronoun but when the verb is not a performance verb, we reply with an object pronoun.
Who is knocking at the door? It is I.
Who is at the door? It is me.
TAKE AWAY:- Patience is the strength of the weak. Impatience is the weakness of the strong. –––Immanuel Kant