Examine the following ALTER command：   SQL> ALTER DISKGROUP dgroup1 UNDROP DISKS；   What is the purpose of the command？（）  

• A、 It cancels all pending disk drops within the disk group.
• B、 It restores disks that are being dropped as the result of a DROP DISKGROUP operation
• C、 It mounts disks in the disk group for which the drop-disk operation has already been co pleted
• D、 It restores all the dropped disks in the disk group for which the drop-disk operation has already been completed
• E、 It adds previously dropped disks back into the disk group