从一开始，盘旋在氢能源开发企业头上的阴云已经不是像这样的动力来源的问题，而是那些棘手的配送和储存原料的难题。
From the beginning, the cloud hanging over the whole hydrogen enterprise has not been the power source as such, but the intractable difficulty of distributing and storing the stuff.
到底用什么来取代石油？ 这个问题显然是人类在这个时代最棘手、但又不得不解决的科技难题。
The vexing question—surely one of the greatest scientific and technological challenges of our time—is what will take petroleum’s place.
它或许是一件事关员工的棘手的情形，或许是一个事关客户的机敏的问题，或者是一个难题，需要你违背自己公司的规定和准则。
It may involve a touchy situation with employees, a tricky problem with a client, or a quandary in interpreting your own company's rules and guidelines.
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