How does a firewall prevent unauthorized access to a computer system?

A$.$ By searching incoming e$-$mail and data files for "signatures" of known viruses and virus$-$like characteristics, and discarding or quarantining them

B$.$ By encoding all intellectual property owned by the organization

C$.$ By filtering incoming data by applying a security policy and using a router to control where data goes

D$.$ By blocking users who send e$-$mail messages that appear as if they came from a trusted source

E$.$ By scrambling e$-$mail messages so that they look like garbled nonsense to anyone who does not possess the code or key