氢是宇宙中最普遍的元素，可以有多种来源，但是它总是倾向于与重元素污染并存这一事实才是关键。
This is the most common element in the universe and so could come from many sources, but the fact that it tends to coincide with heavy-element pollution is telling.
研究还发现利用WWS资源充电的电池汽车和氢燃料电池汽车有助于大幅降低交通领域的污染。
The study also found that battery-electric vehicles and hydrogen fuel-cell vehicles recharged by WWS options would largely eliminate pollution from the transportation sector.
从理论上讲，氢燃料电池驱动的汽车是无污染的，但由于氢难以廉价、大量地生产，这种汽车在普及方面打了折扣。
Vehicles powered by hydrogen fuel cells are, in theory, pollution-free but their popularity has been compromised by the difficulty of generating hydrogen cheaply and plentifully.
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