John is a security manager for the university. He receives a call from law enforcement, telling him that some computers on his network participated in a massive distributed denial of service $($DDoS$)$ attack. John is sure that none of the employees at the university would be involved. What BEST describes this situation?

Question $10$ options:

Machines are infected with crypto$-$viruses

Social Engineering

Machines are part of a botnet

Backdoors on the Machine