An application stores customer profiles in Cosmos DB NoSQL. Queries filter by both country (low cardinality, 50 values) and userId (high cardinality, millions). What is the best partition key?

1. A country
2. ✓ B userId (correct answer)
3. C country_userId (composite synthetic key)
4. D A random GUID

Shortcut: userId offers high cardinality ensuring even distribution. country alone creates hot partitions (only 50 partitions). A synthetic composite key is useful when queries need both fields together.