Why would this happen?

LenoxSoft sends an operational email to all prospects, but finds there are some prospects that did not receive the email.

Why would this happen?
A . The prospects were set to "true" for opted out and set to "false" for do not email.
B. The prospects were on a suppression list used on the operational list email send.
C. The prospects are opted out of the list used on the operational list email send.
D. The prospects were set to "false" for opted out and set to "true" for do not email.

Answer: D