Which music distribution innovation first allowed legal individual song purchases instead of full albums, fundamentally changing the industry's product design and retail strategy?

The music industry has undergone massive transformations with digital technology. One particular innovation revolutionized how we consume music, changing the industry's business model forever. Test your knowledge about this pivotal shift in music retail and distribution that altered everything from product design to consumer behavior and industry revenue streams. Can you identify which music distribution innovation truly disrupted the traditional retail model and forced the entire industry to adapt?

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Apple's iTunes Store (2003)
Napster's peer-to-peer file sharing (1999)
Spotify's streaming service (2008)
Amazon's MP3 Store (2007)