I wasn't 100% sure Mona was "A". What I thought is "A" wanted to get the girls off his or her trail and got Mona to pretend to be "A" so the girls wouldn't be trying to figure out who "A" was anymore.
Also if Mona really is "A" who was the Black Swan at the Ball
and at the end Mona went " I did what you asked me too" or something along those lines, so I just get the feeling that Mona isn't really "A" even though she was in the book *confused*
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