Which type of security control is defense in depth?

A.
Threat mitigation

B.
Risk analysis

C.
Botnet mitigation

D.
Overt and covert channels

Explanation:
BD
Defense in-depth is the key to stopping most, but not all, network and computer related attacks. It’s a concept
of deploying several layers of defense that mitigate security threats.
Source: http://security2b.blogspot.ro/2006/12/what-is-defense-in-depth-and-why-is-it.html