词数:496阅读难度:★★★☆☆建议阅读时间:6分钟HONG KONG—The Malaysian authorities said on Friday that Kim Jong-nam,the half brother of North Korea’s leader,had been killed by VX nerve agent applied to his face.The substance,___1___as a chemical weapon,was kept for decades in the arsenals(兵工厂)of many militaries,including that of the United States.But since the Chemical Weapons Convention came into___2___in 1997,most stockpiles globally have been destroyed.What is a nerve agent? Word Count: 496 Reading Difficulty: ★★★ ☆☆ Suggested Reading Time: 6 minutes HONG KONG-The Malaysian authorities said on Friday that Kim Jong-nam, the half brother of North Korea’s leader, had been killed by VX nerve agent applied to his face.The substance, ___ 1___as a chemical weapon, was kept for decades in the arsenals, including that of the United States.But since the Chemical Weapons Convention Convention came into___2___in 1997, most stockpiles have been destroyed. What is a nerve agent?