They must have kept him under surveillance for a while." U.S.officials have been quoted as saying the operation Monday was launched when it became clear their target was traveling in an area away from heavy population.

VOA: standard.2009.09.15

it becomes a gray area but it should be pretty clear if you can treat something is reversible are irreversible.

这是个灰色区域,但什么可以当作可逆过程处理,而什么要当作不可逆过程,这是相当清楚的。

麻省理工公开课 - 热力学与动力学课程节选

So you want to stay clear out of that area. Otherwise, it's pretty safe.

所以你想远离那个地方。除此之外，这里是相当安全的。

纽约生活 - SpeakingMax英语口语达人