If you compare the A natural minor scale with the C major scale you will notice that they contain the same notes (except starting on a different note). Because of this, these two scales are referred to as being "Relatives". A minor is the relative minor of C major, and C major is the relative major of A minor.
This song is written using notes from the A harmonic scale, and is thus in the key of A minor. On the recording there are four beats to introduce this song.
You can now play the songs on pages 24 and 25 of Supplementary Songbook C.