Answer: b
Explanation: There are two types of firewalls - software programs and hardware-based firewalls. These types of firewalls filter each and every data packet coming from the outside environment such as network; internet so that any kind of virus would not be able to enter in the user's system. In some cases where the firewall detects any suspicious data packet, it immediately burns or terminates that data packet. In short, we can also say that it is the first line of defense of the system to avoid several kinds of viruses.