Scientists first posed this question in the mid-1990s, after researchers came upon a chemical agent that in rodents seemed to reproduce many of caloric restriction's benefits.

科学家首次提出这个问题是在20世纪90年代中期。当时，研究人员偶然发现一种化学药剂，这种药剂在啮齿动物身上似乎可以发挥和热量限制相同的正面效果。

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The chemical agent must be delivered in order to threaten someone, and the threat can be countered.

化学毒剂必须通过投放来实现威胁，而且这种威胁可以防范。

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Lethal chemical Agent Countermeasures reviews some of the measures taken to defend against chemical attack.

致死性化学毒剂对策，综述了防御化学袭击的措施。

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Several people on the ground seem to believe that it was a chemical agent.

WHITEHOUSE: Press Briefing

He said the authorities had not yet ruled out the presence of some sort of biological or chemical agent.

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Hair can also be examined, partly because it will pick up tiny traces of a chemical agent in the atmosphere.

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