After running the command umount /mnt, the following error message is displayed: umount: /mnt: device is busy. What is a

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After running the command umount /mnt, the following error message is displayed: umount: /mnt: device is busy.
What is a common reason for this message?

A. The kernel has not finished flushing disk writes to the mounted device.
B. A user has a file open in the /mnt directory.
C. Another file system still contains a symlink to a file inside /mnt.
D. The files in /mnt have been scanned and added to the locate database.
E. The kernel thinks that a process is about to open a file in /mnt for reading.