Electronic cigarettes are handheld nicotine-delivery devices that, despite a devoted following, are currently swirling in contro

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问题     Electronic cigarettes are handheld nicotine-delivery devices that, despite a devoted following, are currently swirling in controversy. New York is pushing to become the first state to ban the devices, which so far remain【C1】______and mostly unstudied. With cute colors, fruity flavors, clever designs and other options, e-cigarettes may hold too much【C2】______for young people, critics warn, offering an easy gateway to nicotine addiction. But those criticisms clash with【C3】______strong arguments for the value of e-cigarettes. The devices, which are tobacco-free, may be a safer【C4】______to cigarettes, say advocates, who point to thousands of smokers who say they have used e-cigarettes to help them quit.
    Electronic cigarettes【C5】______use a rechargeable battery-operated heating element to vaporize the nicotine in a replaceable pipe. Nicotine is the substance in tobacco that people can become addicted to. Some e-cigarettes are made to look like real cigarettes, cigars or pipes. Others look like pens or USB memory devices. No tobacco is【C6】______in e-cigarettes and there is no smoke either. Instead, as users breathe in, they take in nicotine-filled vapor.
    By isolating nicotine, e-cigarettes should carry far fewer chemical risks than【C7】______cigarettes. Tobacco contains about 5,000【C8】______chemicals with as many as 100,000 more that haven’t yet been known. E-cigarettes【C9】______many of those ingredients. Researchers also found evidence that e-cigarettes reduce desires among smokers, not just for nicotine but also for the need to hold something in their hands and put something in their mouths. As a cigarette-quitting strategy, it’s not that the devices are good for anyone, but they are just better than what they’re meant to【C10】______
    A)alternative B)engaged C)regular D)opaque
    E)involved F)typically G)solely H)appeal
    I)momentum J)equally K)replace L)unregulated
    M)eliminate N)detect O)identified
【C8】

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答案O

解析 空格在名词前面,可能是名词、形容词或分词作定语。空格后提到“仍有10万种以上‘未知的’化学成分”,由此推断空格所在部分讲烟草中已有5000种“已知的”化学成分,identified作定语表示“已鉴定的、已认出的”,符合句意。
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