which Debian installation package files are loaded?

Which of the following files should be edited to select the network locations from which Debian
installation package files are loaded?

A.
/etc/dpkg/dpkg.cfg

B.
/etc/apt/apt.conf

C.
/etc/apt/apt.conf.d

D.
/etc/apt/sources.list

E.
/etc/dpkg/dselect.cfg