You issue this SQL command to perform an online backup of the USER_DATA tablespace：ALTER TABLESPACE user_data BEGIN BACKUP；Next， you issue an operating system command to back up user_data01.dbf， the file that belongs to the USER_DATA tablespace. You then experience an instance failure. Which three statements are true？（）
- A、A
- B、B
- C、C
- D、D
- E、E
正确答案:B,C,D
答案解析:有
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