The following is true about a DDoS attack, except:

a$.$ It's an attempt by an attacker to impede the flow of traffic for normal visitors

b$.$ It's an attempt by an attacker to slow down or shut down targeted servers and systems

c$.$ It's an attempt by an attacker to get behind a system's firewall to steal information

d$.$ It's an attempt by an attacker to control a network of computers to coordinate an attack