Which linguistic phenomenon did Duolingo's product development team specifically leverage when designing their gamified language learning approach?

In the world of language and linguistics, brand names and product development strategies are often influenced by linguistic principles. Companies spend millions researching how consumers perceive and respond to certain sounds, word structures, and linguistic associations. This poll tests your knowledge of how linguistics intersects with product development in the creation of successful language learning applications.

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Spaced repetition systems (SRS) based on the forgetting curve hypothesis
Universal Grammar theory as proposed by Noam Chomsky
The Critical Period Hypothesis for language acquisition
The Sapir-Whorf hypothesis of linguistic relativity