We can use a firewall to protect

We can use a firewall to protect our home network and family from _21_ websites and potential hackers. Firewall plays an important _22_ in controlling, _23_ the connections between the _24_ network of a company and Internet, and safeguarding the security of a certain company.
The firewall used in Internet is not a real wall _25_ a component system of the computer hardware and software, _26_ isolates a private network from a public network. The firewall is used as the protection system of a certain network to _27_ unauthorized users from accessing that network. An enterprise_28_ an intranet(内部网络) that allows its workers to have_29_ to the wider Internet installs a firewall to prevent outsiders from accessing its own private data resources and to control _30_ outside resources its own users have access to.
A、whether
B、which
C、when
D、where
【正确答案】：B