When you install SBS on a computer, that computer becomes the PDC of the SBS domain.
SBS applications can only be installed on the SBS PDC.
You can install Windows NT Server as a BDC and/or standalone server in the SBS domain, but these computers can not run the SBS applications.
When the PDC goes down, the BDC can validate users,
but this is likely to be as useless as ___ __ _ ____ (fill in the blanks).