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The database Is configured in ARCHIVELOG mode. The database needs to be up 24 X 7. You want to perform user managed backup for the data files of the HR_DATA tablespace. To accomplish the task， you issued the following command：  SQL> ALTER TABLESPACE hi_data BEGIN BACKUP；   Which two statements are true in this scenario？（）  
A: No transaction on the tablespace is allowed but you can perform queries.
B: The tablespace will automatically come out of backup mode when the file copy is complete.
C: The checkpoint change number is frozen in headers of the data files until the file is removed from backup mode.
D: The database writes the before image of an entire block to the redo stream before modifying. block tor the first time.

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出自：Oracle认证

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