FIR is _______ .
A.an airspace of defined dimensions within which flight information control service is provided
B.an airspace of defined dimensions within which ATC service and flight information service are provided.
C.an airspace of defined dimensions within which flight information service and alerting service are provided.
D.an airspace of defined dimensions within which ATC service, flight information service and alerting service are provided.
正确答案是C