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.