Which of the following represents the BEST method of securing physical access to a network backbone?

Which of the following represents the BEST method of securing physical access to a network backbone?

A.
Installing biometric security in all server room doorways.

B.
Forcing all remote users connecting to the network externally to use encrypted SSL VPN.

C.
Installing hardened deadbolts in all server room doorways.

D.
Requiring users to authenticate network accounts via complex passwords and smart cards.