Aviation and Airport Security Practice Test.

The primary agency responsible for protecting the nation's mail system is the USPIS, or United States Postal Inspection Service. The USPIS is responsible for investigating and preventing criminal activity that may involve the mail, such as mail theft, fraud, and illegal drug trafficking. This is their primary focus, whereas the other options listed (FBI, Interpol, and USNCB) have broader responsibilities and may not necessarily prioritize protecting the mail system. The FBI (Federal Bureau of Investigation) has a wide range of duties related to national security and law enforcement, while Interpol (International Criminal Police Organization) is an international law enforcement agency. The USNCB (United States National Central Bureau of Interpol) is a part of the FBI and manages communication and cooperation with Interpol on behalf of the US. Therefore, although these agencies may play a role in protecting the nation's mail system, it