What changed the course of the peloponnesian war?

Amid the whole course of the Peloponnesian War there were not more than three or four hoplite clashes of the old style. Rather, hired soldiers, daintily equipped skirmishers, mariners, and attack engineers filled that void. All were costly. And– awful for both sides– all obviously were unequipped for closure an engagement conclusively during a time's obliteration or embarrassment of a foe's powers in the field.

The answer is letter c