What does the SCN represent？（）  

• A、 The system change number， which is a point in time relative to transactions within a given database.
• B、 A number that represents time. Thus， at 1300 hours， the SCN is the same on all databases. 
• C、 The security change number， which represents the security code that is needed to access any database structure.
• D、 A conversion factor that converts internal database time to external clock time.
• E、 UTC time in the database，providing a standardized way of tracking time in Oracle.