外骨骼系统一直是漫画书和未来风格电影中不可缺少的元素之一，但现在，它们已不再是一种科学幻想了。

Exoskeletons have long been a fixture of comic books and futuristic movies, but they aren't science fiction any more.

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过去期货叫商品，是经济中不可缺少的元素。

Futures used to be called commodities, the irreducible building blocks of the economy.

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铜是生物体不可缺少的一种元素，在细胞内把铜转运到含铜的蛋白质是细胞正常代谢的基本要求。

Copper is an essential element for living organism, and transporting copper to copper -dependent proteins is a basic requirement for normal cellular metabolism.

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