Duolingo is a widely used foreign language app, with 300 million users in 2018. While there is a free version of the app, a premium Duolingo subscription eliminates advertisements during language practice and offers new quizzes with the ability to save lessons for offline use. Explain how Duolingo uses the hurdle method to allow its users to self-sort into “free” users (for whom Duolingo still earns revenue, through advertising) and “premium” users, who pay to use the enhanced, ad-free app.