$21.$ Which statements regarding a DoS $($Denial$-$of$-$Service$)$ attack are TRUE $($More than one statement applies$)?$

a$.$

It may result from malfunctioning software.

b$.$

Specific types of DoS attacks include DDoS $($Distributed DoS$)$ attack, DRDoS $($Distributed Reflection DoS$)$ attack, amplified DRDoS attack, PDoS $($Permanent DoS$)$ attack and Friendly DoS attack.

c$.$

A DoS $($Denial$-$of$-$Service$)$ attack occurs when a legitimate user is unable to access normal network resources, such as a web server, because of an attacker$$s intervention.

d$.$

This incursion is a relatively complex attack to launch.