The Difference Between an Overt and a Covert Bioterrorism Event

The difference between an overt and a covert bioterrorism event is that a(n) :

A) covert event is one that is carried out against undercover agents.
B) covert event is announced,and an overt event is not announced.
C) overt event is announced,and a covert event is not announced.
D) overt event is one that is carried out in the air.