Which of the Following Best Describes a Firewall

Which of the following best describes a firewall?

A) a software and hardware system that prevents outsiders from accessing a company's internal network
B) a tool that searches Web pages containing the user's search terms and then displays pages that match
C) a private network of internal Web sites and other sources of information available to a company's employees
D) an Internet connection allowing outsiders limited access to a firm's internal information system
E) an extranet site that gives customers access to different product lines