What is the best security policy concerning this setup?

A large mobile telephony and data network operator has a data center that houses network elements. These are essentially large computers running on Linux. The perimeter of the data center is secured with firewalls and IPS systems.

What is the best security policy concerning this setup?
A . Network elements must be hardened with user ids and strong passwords. Regular security tests and audits should be performed.
B . As long as the physical access to the network elements is restricted, there is no need for additional measures.
C . There is no need for specific security measures on the network elements as long as firewalls and IPS systems exist.
D . The operator knows that attacks and down time are inevitable and should have a backup site.

Answer: A