An Amazon DynamoDB table uses a Global Secondary Index (GSI) to support read queries. The primary table is write-heavy, whereas the GSI is used for read operations. Looking at Amazon CloudWatch metrics, the Developer notices that write operations to the primary table are throttled frequently under heavy write activity. However, write capacity units to the primary table are available and not fully consumed.
Why is the table being throttled?

A. The GSI write capacity units are underprovisioned
B. There are not enough read capacity units on the primary table
C. Amazon DynamoDB Streams is not enabled on the table
D. A large write operation is being performed against another table