Which pricing strategy was pioneered by Duolingo in 2011 that dramatically disrupted the language learning software market?

In the language learning and linguistics industry, pricing strategies often reflect the perceived value and accessibility of language acquisition tools. From premium immersion software to budget phrasebooks, companies employ various approaches to monetize language products. This poll explores an interesting pricing phenomenon that emerged in the 2010s as digital language learning platforms revolutionized the market.

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Freemium model with ad-supported basic content and premium subscription for advanced features
Dynamic pricing based on language popularity (higher prices for common languages like Spanish, discounts for less common ones)
Microtransactions for individual grammar lessons and vocabulary sets
Pay-per-fluency model where users only pay after demonstrating proficiency in a language level