Many opponents are c-betting most of their pre-flop range there. In this situation, I look to my HUD and see what their c-bet percentage is, and if they've folded their c-bets to a raise. If their c-bet percentage is low, I want to raise because they likely have a piece of the flop and won't fold. If their c-bet percentage is high, but they call when their bets get raised, I want to raise for the exact same reason. Otherwise I consider just flatting.