高等学校分类是一个世界性的难题。
Classification of HEIs is a difficult problem throughout the world.
水下结构物的安全拆除是一个世界性的难题。
Safe Dismantlement of underwater structures is a worldwide hard work.
暴雨灾害是一种世界性的自然灾害，同时也是一个世界性的难题。
The rainstorm disaster is one kind of worldwide natural disasters, simultaneously it is also a worldwide difficult problem.
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