Which of the following can be thought of as the simplest and almost cheapest type of firewall?

A.
stateful firewall

B.
hardware firewall

C.
PIX firewall

D.
packet filter

E.
None of the choices.

Explanation:
The simplest and almost cheapest type of firewall is a packet filter that stops messages with inappropriate
network addresses. It usually consists of a screening router and a set of rules that accept or reject a message
based on information in the message header.