What controversial pricing strategy did Duolingo implement in 2017 that dramatically increased its revenue while maintaining its 'language learning is free' promise?

Duolingo revolutionized language learning with its gamified approach and freemium model. The company transformed how people access language education and created a new market segment. This poll explores how Duolingo's pricing strategy contributed to its massive growth and market penetration, challenging traditional language learning products and services. Test your knowledge about this linguistic product phenomenon!

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The introduction of Duolingo Plus with streak protection and offline access
Implementation of a 'pay-per-language' model where only the first language was free
A mandatory monthly subscription after completing the first 30 lessons
Charging for certification tests while keeping lessons free