What does the command mount -bind do?

What does the command mount –bind do?

What does the command mount –bind do?
A . It makes the contents of one directory available in another directory
B . It mounts all available filesystems to the current directory
C . It mounts all user mountable filesystems to the user’s home directory
D . It mounts all file systems listed in /etc/fstab which have the option userbindset
E . It permanently mounts a regular file to a directory

Answer: A