The f(x) girls kicked off the week with a fierce comeback stage on Music Bank, and now they're returning with their sassy charisma on today's Music Core. The group's initial set of singles were definitely catchy and charming, but their new track NU ABO is showing a stronger and fiercer side to the group. And let's not forget the sexy! With the girls lifting their shirts throughout the entirety of the performance, it seems as though they're ready to say "bye bye" to their cute image. Just check out Krystal at 1:00 and 1:51 as she reveals her red bra. After checking out their performance today, wouldn't you agree that this new f(x) is pretty awesome? The Mr. Boogie performance never happened, it was just a rumor. http://www.youtube.com/watch?v=ySM7mZOuqTE