Why malware that uses virtualization techniques is difficult to detect?

Why malware that uses virtualization techniques is difficult to detect?

A.
The malware may be implementing a proxy server for command and control.

B.
A portion of the malware may have been removed by the IDS.

C.
The malware may be using a Trojan to infect the system.

D.
The malware may be running at a more privileged level than the antivirus software.