Which pricing innovation by Duolingo fundamentally changed the language learning market and became their key differentiator?

In the world of language learning apps and platforms, pricing strategies have evolved dramatically over the past decade. From traditional one-time purchases to subscriptions and the freemium model, companies have experimented with various approaches to monetize their products while balancing accessibility and profitability. This trivia question focuses on a particularly innovative pricing strategy that revolutionized how language learning products are marketed and consumed.

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The 'completely free with ads' model that made premium language education accessible to everyone
Their 'pay-per-lesson' micropayment system that allowed users to purchase only the specific language modules they needed
The 'fluency guarantee' model where users only pay if they achieve certified proficiency within a set timeframe
Their 'corporate sponsorship' approach where major companies subsidize certain language courses relevant to their global markets