What is Peter Smith talking about?

You went to great lengths to install all the necessary technologies to prevent hacking attacks, such as expensive firewalls, antivirus software, antispam systems and intrusion detection/prevention tools in your company’s network. You have configured the most secure policies and tightened every device on your network. You are confident that hackers will never be able to gain access to your network with complex security system in place.
Your peer, Peter Smith who works at the same department disagrees with you. He says even the best network security technologies cannot prevent hackers gaining access to the network because of presence of “weakest link” in the security chain.
What is Peter Smith talking about?

A.
Untrained staff or ignorant computer users who inadvertently become the weakest link in your security chain

B.
Continuous Spam e-mails cannot be blocked by your security system since spammers use different techniques to bypass the filters in your gateway

C.
“zero-day” exploits are the weakest link in the security chain since the IDS will not be able to detect these attacks

D.
“Polymorphic viruses” are the weakest link in the security chain since the Anti-Virus scanners will not be able to detect these attacks