The /prod/inv file system needs to be unmounted so you can take it offline for maintenance. When you try and unmount it, you receive the following error message:

umount: /prod/inv busy

One or more users are still using it.

Which command should you use to discover who is using the file system?

A.
who -q

B.
who /prod/inv

C.
fuser -ck /prod/inv

D.
fuser -cu /prod/inv