The correct answer is “Costs increased because airlines had to improve security.”
After 9/11 the US Congress passed the Aviation and Transportation Security Act that created the Transportation Security Administration. This made federal employees responsible for the airport security screening and other procedures were implemented for passengers and their baggage, such as metal detectors, x-rays for carry-on bags, explosive detection system, prohibition of carry-on liquids. Presence of police outside the airports also increased and cooperation between airlines and security officers were implemented.