PREDICATE NOUN: You are who?
" Whom" is correct if it is a direct object or an object of a preposition.
Direct Object: The man whom he hit is suing.
Object of Preposition: To whom is the message directed?
Part of the confusion over " who" and " whom" arises because the words often occur in complex sentences where it is difficult to see the function of the word in the sentence.
Give the message to whoever answers the door.
(The correct word here is " whoever" because it is the subject of " answers." People tend to use " whomever" here because it comes after a preposition, but the object of the preposition is the entire clause " whoever comes to the door.")
(See more at sentence structure.)