What does the command mount -a do?

What does the command mount -a do?

A.
It mounts all available filesystems onto the current directory.

B.
It shows all mounted filesystems.

C.
It mounts all user mountable filesystems for the current user.

D.
It mounts all filesystems listed in /etc/fstab which have the option auto set.

E.
It mounts all filesystems listed in /etc/fstab which have the option no auto set.