Which retail innovation first introduced the 'free-to-play with microtransactions' model that has become dominant in mobile and many PC/console games?

The gaming industry has seen numerous retail innovations that transformed how players purchase and access games. From physical cartridges to digital marketplaces, the evolution of game distribution has dramatically changed the player experience and industry economics. Test your knowledge about a pivotal retail innovation that forever changed how games reach consumers, creating new revenue models and expanding the gaming ecosystem.

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Valve's Steam platform (2003)
Korean and Chinese MMORPGs (2001-2005)
Apple's App Store (2008)
Facebook gaming platform with Zynga's FarmVille (2009)