The Independent-Minded Greek Poleis Tended to Unite Only When Confronted

The independent-minded Greek poleis tended to unite only when confronted by what sort of political or military conditions?

A) A rival polis showed unexpected economic prosperity.
B) They wished to spread their empire to Egypt.
C) They were confronted with threats from Roman imperialism.
D) They viewed a particular polis or external enemy as a common threat.
E) They wanted to plunder foreign lands much like pirates would do.