053-660

You want to configure the Flashback Database feature and retain flashback logs for three days.
Presently the database is open and configured in ARCHIVELOG mode.
The following steps must be performed in the correct sequence to do this:
1. Set the retention target:
SQL> ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=4320;
2. Enable Flashback Database:
SQL> ALTER DATABASE FLASHBACK ON;
3. Mount the database:
SQL> STARTUP MOUNT EXCLUSIVE
4. Shutdown the instance:
SQL> SHUTDOWN IMMEDIATE
5. Open the database:
SQL> ALTER DATABASE OPEN;
Choose the correct sequence.
A. 4, 1, 3, 2, 5
B. 4, 3, 1, 2, 5
C. 2, 4, 3, 5, 1
D. 4, 2, 5, 3, 1

　　启用闪回数据库特性。数据库实例必须在 open 或 mount 状态。但是在数据库打开状态时候可能会由于内存的原因命名执行失败，可以先关闭实例，再启动。 DB_FLASHBACK_RETENTION_TARGET 参数用来确定希望保持的闪回时间窗口，单位是分钟。
　　ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=4320; # 3 days
　　然后使用 ALTER DATABASE FLASHBACK ON;在数据库级别开启闪回数据库特性。最后 open 数据库
注意：此处并不是必须要将数据库重启到独占 mount，按照题意，这样操作是可行的。