A.It provides each session with its own view of the database at a different point in the past. 
B.It enables you to execute scalable applications on a clustered database without having to partition the users or the database tables. 
C.It lets you dynamically reassign memory in your database buffer cache to different block buffer sizes. 
D.It allows you to add new sites to multimaster replication environment without quiescing the master definition site.

A.Global indexes are rebuilt automatically at the end of the DDL operation thereby avoiding problems with the UNUSABLE status. 
B.You do not have to search for invalid global indexes after the DDL command completes and rebuild them individually. 
C.Global indexes are maintained during the operation of the DDL command and therefore can be used by any concurrent query. 
D.Global indexes remain intact and available for use by data manipulation language （DML） statements even for sessions that have not enabled the skipping of unusable indexes.