What option can be used with the shutdown command to send a warning message alerting users that the system will be shut down?

A.
shutdown -k

B.
shutdown -w

C.
shutdown -a

D.
shutdown -c

Explanation:
About shutdown

Turn off the computer immediately or at a specified time.

Syntax

shutdown [-a][-t sec][-krhnfFc][time][warning-message]
-a Use /etc/shutdown.allow.
-t sec Tell init to wait sec seconds between sending processes the warning and the kill signal, before changing to another runlevel.
-k Don’t really shutdown; only send the warning messages to everybody.
-r Reboot after shutdown.
-h Halt after shutdown.
-n Don’t call init to do the shutdown but do it ourselves. The use of this option is discouraged, and its results are not always what you’d expect.
-f Skip fsck on reboot.
-F Force fsck on reboot.
-c Cancel an already running shutdown. With this option it is of course not possible to give the time argument, but you can enter an explanatory message on the command line that will be sent to all users.
time When to shutdown.
warning-message Message to send to all users.