In this pattern you are working in rounds. When working the sc/dc alternating stitch you always work a Sc in the top of a previous rnds Dc and a Dc in the top of a previous rnds Sc. This is a little confusing when beginning a round. Normally you would make your first stitch in A, but because you need to work your first stitch (always a Sc) in the first Dc of the previous round, you need to begin in stitch B