Which metric is most commonly used by subscription-based language learning apps to measure the effectiveness of their product design and user engagement?

In the competitive world of language learning applications, product marketing strategies rely heavily on understanding user engagement and retention metrics. Companies like Duolingo, Babbel, and Rosetta Stone have pioneered different approaches to measuring success in this specialized market. Test your knowledge about how these language learning products track and measure their effectiveness!

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Daily Active Users to Monthly Active Users ratio (DAU/MAU)
Cost Per Acquisition (CPA)
Click-Through Rate (CTR) on advertisements
Number of languages offered