Key Signature?
 

Could someone tell me what key signature this song is in?

musescore(dot)com/user/113114/scores/125533

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Thanks!

It's in C Major.

Just wondering, but why isn't it A minor? Don't both key signatures have 0 sharp and flats? What's the difference between the two?

It depends on the feel of the song. Its chord progression, which notes important phrases begin and end on, and the general melodic structure generally determine the key. Often a minor key will have a raised seventh note (in a minor, a G would become a G#) and sometimes sixth note (F becomes F# in a minor). However, BD has a more extensive knowledge of music theory than I, and can probably give you a better explanation than I just have.