Which of the following files should be edited to change the default background for it?

An administrator has just set up the X Display Manager as the default display manager. Which of
the following files should be edited to change the default background for it?

A.
/etc/X11/xdm/Xsetup

B.
/etc/X11/prefdm

C.
/etc/X11/xorg.conf

D.
/etc/X11/xdm.conf