3. For computer systems, the difference between anti-virus software and a firewall is that ... (a) ...firewalls will only block malware programs when they try to enter a computer system but anti-virus software will counteract malware programs already inside the computer. (b) ... anti-virus software will only block malware programs when they try to enter a computer system but firewalls will counteract malware programs already inside the computer. (c) ... anti-virus software is normally free of charge but firewalls are NOT free of charge. (d) ... there is no significant difference between firewalls and anti-virus software