A—air traffic control system J—safety precaution measureB—armed police K—safety control deviceC—crime prevention L—security command centerD—entry requirement M—security serviceE—international criminal police organization N—security control centerF—level of security O—security personnelG—picket line P—valid documentsH—police station Q—security monitoring and controlI—patrolling vehicleExamples：(M)保安服务 (G)警戒线