King Kong : Fantasy or Sci-Fi?

Like the title says, is this movie a fantasy movie or sci-fi? I know it has a little of both, but if I were to put it in my library, and only had one copy, where should this go: fantasy or sci-fi?

Thank you for your answers.

Logan: I don't know!
Rogue: You don't know, or you don't care?
Logan: Pick one. X-Men

King Kong has always been a fantasy to me. While they do try to explain the existence of prehistoric creatures on an island in the 30s with the backstory/other media related to the film, its not actually explained in the film. That's the beauty of it, and what sets a film like King Kong apart from its monster movie contemporaries. It may be sci-fi when you look at it in certain ways, but it presents itself like a fantasy in the same way Star Wars is essentially a fantasy.

I'd put it in the fantasy/adventure box.