Which of the following allows a hidden backdoor to be used for access to workstations on the Internet?

Which of the following allows a hidden backdoor to be used for access to workstations on the
Internet?

A.
Firmware

B.
Rootkits

C.
SQL injection

D.
Cross-side scripting

Explanation:
http://www.spamlaws.com/how-rootkits-work.html