Which of the following is solely designed to avoid detection by an antivirus program by using the underlying operating system to its advantage?

Which of the following is solely designed to avoid detection by an antivirus program by using the
underlying operating system to its advantage?

A.
Rootkit

B.
Virus

C.
Trojan

D.
Worm

Explanation:
http://www.webopedia.com/TERM/R/rootkit.html