单项选择题 （）means the level for which further specific protective security measures shall be maintained for a limited period of time when a security incident is probable or imminent, although it may not be possible to identify the specific target.

单项选择题 （） means the level for which appropriate additional protective security measures shall be maintained for a period of time as a result of heightened risk of a security incident.

单项选择题 （） means the level for which minimum appropriate protective security measures shall be maintained at all times, threats and exchanging such information with appropriate Contracting Governments.