Which controversial marketing strategy helped Duolingo dramatically increase user engagement and become the most downloaded education app worldwide?

In the competitive world of language learning apps, marketing strategies often leverage linguistic psychology to attract and retain users. This poll tests your knowledge about one of the most successful marketing pivots in language learning technology from the past decade, which revolutionized how language apps advertise their products and engage with potential customers.

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The guilt-inducing 'Duolingo Owl' notifications that created memes about the mascot stalking users who miss lessons
Their 'Language Fluency Guarantee' promising refunds if users don't achieve conversational fluency within 3 months
The 'Native Speaker Challenge' advertising campaign claiming their app produces better pronunciation than native speakers
Their 'Linguistic Immersion' technology that supposedly replicates living abroad through VR experiences