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