ADJ-GRADED
(污渍)顽固的，难以去除的;(问题)棘手的，难以解决的
A stubborn stain or problem is difficult to remove or to deal with.
例句
This treatment removes the most stubborn stains...
这个方法能清除大多数顽固污渍。
The first and most stubborn problem was that of reductions in the number of aircraft.
最首要也是最棘手的问题是飞机数量的减少。