(1) TSA also recognizes that transportation assets, such as airplanes and tunnels, are part of larger systems, such as the national aviation system or a mass transit system. Taken together, all the individual transportation systems form the national transportation system (i.e., a "system of systems"). The behavior of transportation systems cannot be fully explained by confining observations to individual cars, vessels, and aircraft or fixed infrastructure. An attack on a specific asset must be analyzed for how it will impact the larger transportation system within which it resides. Impacts to a specific transportation system may then have ripple-effects on other transportation systems that could result in cascading failures.
(2) In December 2005, we made a risk-based choice to change our operations to better meet this threat. Before the change, our screening workforce was spending a lot of time searching for items that no longer posed a significant risk - items that would no longer be useful to terrorists intent on taking control of an aircraft. Based on analysis of threats, vulnerabilities and consequences, we removed the most innocuous of these from our list of items that are prohibited beyond the checkpoint. The time and labor we used to spend detecting them has been shifted to finding explosives instead.
(3) The nature of the current threat has changed since September 11, 2001. The deployment of measures like hardened cockpit doors, the Federal Flight Deck Officer Program, a vastly expanded Transportation Security Inspector Program, and others have greatly reduced the risk of an attack similar to those of September 11. Today, explosives pose the greatest risk to our transportation systems.
(4) Delivering the best security we can means using risk to guide our decisions - maximizing our resources by directing them where they are most needed. Working with our network of federal and industry partners, we identify the areas of greatest risk throughout transportation systems and act to prevent attacks and mitigate their potential consequences. To meet future threats, we are ensuring that we maintain surge capacity to respond when and where they emerge.
A) 4 - 3 - 2 - 1
B) 3 - 4 - 1 - 2
C) 2 - 3 - 4 - 1
D) no correction is necessary