Which of the following answers below best describes Botnet malware resulting in a denial$-$of$-$service $($DoS$)$ attack?

a$.$ causes the CPUs of the server on which it's planted to work extra hard and consume more energy.

b$.$ designed to encrypt user data until the server owner pays a ransom fee. But even after paying the fee, there's no guarantee that the data can be recovered

c$.$ deploys a trojan horse to the system when another program is installed

d$.$ causes a server to join a botnet that is controlled by a remote attacker.