A.It is NOT possible to switch unless no active transaction exist in UNDO1. 
B.It is possible to switch to UNDO2； but current active transactions will abort. 
C.It is possible to switch to UNDO2； current active transactions will be automatically migrated to UNDO2. 
D.It is possible to switch to UNDO2； only current active transactions will continue to execute inside UNDO1.

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.