What database implementation would better fit this scenario, keeping costs as low as possible?

You need a persistent and durable storage to trace call activity of an IVR (Interactive Voice Response) system. Call duration is mostly in the 2-3 minutes timeframe. Each traced call can be either active or terminated. An external application needs to know each minute the list of currently active calls, which are usually a few calls/second. Put once per month there is a periodic peak up to 1000calls/second for a few hours. The system is open 24/7 and any downtime should be avoided.

Historical data is periodically archived to files. Cost saving is a priority for this project.

What database implementation would better fit this scenario, keeping costs as low as possible?
A . Use RDS Multi-AZ with two tables, one for -Active calls" and one for -Terminated ca lls". In this way the "Active calls_ table is always small and effective to access.
B . Use DynamoDB with a "Calls" table and a Global Secondary Index on a "lsActive"’ attribute that is present for active calls only In this way the Global Secondary index is sparse and more effective.
C . Use DynamoDB with a ‘Calls" table and a Global secondary index on a ‘State" attribute that can equal to "active" or "terminated" in this way the Global Secondary index can be used for all Items in the table.
D . Use RDS Multi-AZ with a "CALLS" table and an Indexed "STATE* field that can be equal to ‘ACTIVE" or -TERMINATED" In this way the SOL query Is optimized by the use of the Index.

Answer: A