What was the impact of the september 11th attacks on the tourism and travel industries? costs increased because airlines had to build safer planes. overall travel increased because people refused to be afraid. revenue increased because more people traveled internationally. costs increased because airlines had to improve security.

The airline industry was severely affected by 9/11 due being the weapon used in the attacks; then, the U.S. government prohibited the commercial fleet for 3 days, causing massive industry losses.

Congress also executed changes to improve security, so the "TSA" (Transportation Security Administration) and "Homeland Security Dpmt." were created.

Thus, extensive security precautions on commercial airlines were established, and they came at a cost, like the increment in the time cost of travel, and the decline of potential travelers.

So, the answer is:

"Costs increased because airlines had to improve security".

assessment is the missing word