Several employees return to work the day after attending an industry trade show. That same day, the security manager notices several malware alerts coming from each of the employee`s workstations. The security manager investigates but finds no signs of an attack on the perimeter firewall or the NIDS. Which of the following is MOST likely causing the malware alerts? 

Answer options:

A. A worm that has propagated itself across the intranet, which was initiated by presentation media
B. A fileless virus that is contained on a vCard that is attempting to execute an attack
C. A Trojan that has passed through and executed malicious code on the hosts
D. A USB flash drive that is trying to run malicious code but is being blocked by the host firewall

A