难度测验 power test ; altitude test ; difficulty test

难燃性试验 fire resistance test

测验难度 test difficulty

难度逐步提高的测验 scaled test

极为难得的社会经验 hard-earned social experiences

实验中遇到的困难 Difficulty in Experiment

虫子试验室的大灾难 Break-Out at the Bug Lab

一个很难的测验 a difficult quiz

最艰难的经验 a most trying experience

最艰难的体验 a most trying experience