What is the first thing that must be done to implement network security at a specific site?

What is the first thing that must be done to implement network security at a specific site?

A.
Hire a qualified consultant to install a firewall and configure your router to limit access to known traffic.

B.
Run software to identify flaws in your network perimeter.

C.
Purchase and install a firewall to protect your network.

D.
Install access-control lists in your perimeter routers, so you can ensure that only known traffic is getting through your router.

E.
Design a security policy.