Which language learning subscription service pioneered the use of 'adaptive spaced repetition' algorithms as its core learning methodology?

In today's digital age, language learning has been revolutionized by subscription-based platforms that offer comprehensive tools for mastering new tongues. These services combine various pedagogical approaches with technological innovations to create engaging learning experiences. But how well do you know the distinctive features that set these popular platforms apart? Test your knowledge of the language learning subscription landscape with this trivia question!

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Duolingo
Memrise
Babbel
Anki