Which subscription model helped Brilliant.org become one of the most successful mathematics learning platforms?

Educational technology has revolutionized how mathematics is taught and learned worldwide. Different subscription models have emerged to serve various segments of learners. This trivia tests your knowledge about one of the most successful subscription strategies in mathematics education platforms that has influenced the entire EdTech industry.

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Freemium with premium monthly tiers based on difficulty levels
One-time purchase with lifetime access to all content
Pay-per-problem model where users only pay for problems they solve
Classroom-only licensing with no individual subscriptions