rarefied

高海拔地区的空气会变得稀薄，因此登山者需要携带氧气罐。

At high altitudes, the air becomes ___rarefied, so climbers need to carry oxygen tanks.

这位教授只参加那些最高端学术圈的研讨会。

The professor only attends seminars in the most ___rarefied academic circles.

这家拍卖行服务的是一群精英客户，他们觉得六位数只是个合理的起拍价。

The auction house catered to a ___rarefied clientele who considered six figures a reasonable opening bid.