Which streaming innovation first introduced the 'pay-per-stream' model that revolutionized how artists earn revenue in the digital music economy?

The digital era has dramatically reshaped how music reaches consumers. From vinyl records to digital streaming platforms, the music retail landscape has undergone seismic shifts in distribution models, revenue streams, and customer engagement. This trivia question focuses on a pivotal market trend that has fundamentally altered how music products are consumed and monetized in the modern era.

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Spotify's Premium subscription tier (2008)
Apple's iTunes Store pay-per-download system (2003)
SoundCloud's monetization program 'On SoundCloud' (2014)
YouTube Music's Content ID system (2007)