After performing a clean shut down of the database instance for maintenance， you mount the database andthen execute a command to open the database：SQL> ALTER DATABASE OPEN； Which two statements are true（）
A. The online redo log files and online data files are opened.
B. All the online data file headers are checked for consistency.
C. Instance recovery is performed before opening the database.
D. The path and existence of all the log file members are checked
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