11、Which three are true about UNDO data?
A) It is used to roll back failed transactions.
B) It is used to rewind a database by using FLASHBACK DATABASE.
C) It is used to recover dropped tables by using FLASHBACK TABLE … TO BEFORE DROP.
D) It is used to enable ROLLBACK statements to roll back uncommitted updates.
E) It is used to provide read consistency.
F) It is used during online backups to guarantee their consistency.
Answer:ADE
原文：http://blog.51cto.com/13854012/2141302