You will be given a definition of a task first, then some input of the task.
In this task, we ask you to write an answer to a question that involves event "frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to Saturday market" usually happens every few weeks/months, etc. Note that a lot of the questions could have more than one correct answer. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.

Sentence: One result was two NSC-led interagency trips to Persian Gulf states in 1999 and 2000. 
Question: How often are trips made to the Persian Gulf by the interagency of the NSC?
Output:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
once a year.