Your API has free (100 req/min) and paid (10,000 req/min) tiers. How should you implement this?

1. A Use a centralized rate limiting service with tier-specific quotas stored in a fast key-value store like Redis, keyed by the client's API token.
2. B Deploy physically isolated clusters of API servers for each tier, using DNS-level routing to direct free and paid users to their respective hardware.
3. C Embed the specific rate limit values directly into the application's source code to ensure that every request undergoes strictly local validation logic.
4. D Enforce a uniform rate limit for all users across the system to maintain fairness and prevent any single client from monopolizing the shared thread pool.