Which file would you edit to achieve this?

Your senior administrator asked you to change the default background of his machine, which uses XDM. Which file would you edit to achieve this?

A.
/etc/X11/xdm/Xsetup

B.
/etc/X11/xdm.conf

C.
/etc/X11/xdm/Defaults

D.
/etc/X11/defaults.conf