Which innovative marketing strategy did Duolingo implement that transformed the language learning app industry by creating a viral mascot-centered campaign?

In the competitive world of language learning apps, innovative marketing strategies often make the difference between success and obscurity. Language acquisition platforms have pioneered unique approaches to customer engagement, brand recognition, and user retention. This trivia tests your knowledge about a groundbreaking marketing strategy that revolutionized how language learning products are positioned in the market. Can you identify the technique that transformed a linguistic tool into a cultural phenomenon?

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The passive-aggressive green owl meme campaign that personified the app's reminders through humorous threats about missed lessons
The 'language immersion simulation' campaign where users received spontaneous phone calls from native speakers
The 'linguistic tourism' campaign that offered virtual reality experiences in countries speaking the target language
The 'vocabulary twin' strategy that matched users with celebrity lookalikes who spoke their target language