Answer :
[tex]\bf \underset{dependent}{\stackrel{\textit{songs I can download}}{S(M)}}~~=~~\underset{independent}{\stackrel{\textit{how much memory I have}}{M}}[/tex]
the memory is the independent, because that can vary at will, so it's independently varying at will, whilst the amount of songs are limited to how much memory there's in the CD, so the songs are dependent on the memory availability.