Which retail technology innovation first allowed customers to purchase and download individual digital music tracks legally, fundamentally disrupting the album-based physical music retail model?

The music industry has undergone dramatic transformations in how consumers purchase and experience music. From physical record stores to digital downloads to subscription streaming services, retail technology has revolutionized music consumption. This trivia question tests your knowledge about a pivotal technological advancement that changed music retail forever.

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Apple's iTunes Store (2003)
Napster's P2P service (1999)
Amazon MP3 Store (2007)
Microsoft's Zune Marketplace (2006)