A minor and G#
The following pieces are in the key of A minor. The key of A minor is the relative minor of C. This means it shares the same key signature (no sharps and flats). Every major key has a relative minor. This also means that A minor uses the same notes as C major but the scale starts on A instead of C.
There are different kinds of minor scales including natural minor, harmonic minor and melodic minor scales. An A harmonic minor includes a raised 7th degree (G) so we need to add a G#.
The following piece is in A minor and includes G# on the third string.