During Duolingo's initial product launch, which innovative strategy did they use that significantly contributed to their rapid early growth?

Language learning apps have transformed how we acquire new languages, with some achieving remarkable success through innovative product launch strategies. Duolingo, one of the most successful language learning platforms, implemented a specific tactic during its initial release that helped it achieve viral growth. Test your knowledge of language product marketing with this question about what made Duolingo's launch strategy uniquely effective in the language learning market.

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The waitlist/invitation system that created exclusivity and word-of-mouth marketing
Celebrity language teachers who promoted the platform on social media
Free lifetime subscriptions to the first 10,000 users
Regional language specialization before expanding to multiple languages