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  3. It took decades to prove that cigarette smoking causes cancer, heart disease, and early death. It took【C1】________years to estab

It took decades to prove that cigarette smoking causes cancer, heart disease, and early death. It took【C1】________years to estab

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问题     It took decades to prove that cigarette smoking causes cancer, heart disease, and early death. It took【C1】________years to establish that secondhand smoke also kills. Now scientists are worried about another cigarette-related phenomenon: thirdhand smoke. It’s real, and it’s found everywhere. Without knowing it, indoor smokers have left a toxic【C2】________that continues years after their last cigarette was extinguished.
    Researchers now know that thirdhand smoke combines with some substances to create new compounds. Thirdhand smoke【C3】________and settles with dust,【C4】________down to carpeting and furniture【C5】________, and makes its way deep into the material in paneling and wall. It【C6】________in the hair, skin, clothing, and fingernails of smokers—【C7】________a mother who doesn’t smoke【C8】________her kids, smokes outside, then comes inside and holds the baby is exposing that child to thirdhand smoke. The new compounds are【C9】________to clean up, have a long life of their own, and many may cause cancer. And the contamination becomes more toxic【C10】________time.
    No one knows, in this relatively new field of research, how long the compounds created by smoke and environmental pollutants last. "In homes where we know no smoker has lived for 20 years, we’ve still found【C11】________of these compounds in dust, in wallboard," says a scientist. But scientists do know that babies and children are most【C12】________to the toxic effects of thirdhand smoke. They【C13】________on rugs, fall asleep on carpets, and teethe on furniture, all of which could be【C14】________with thirdhand smoke.
    Researchers aren’t just worried about the risk of cancer. Thirdhand smoke could be【C15】________other health problems. Those who move into houses or apartments【C16】________owned by smokers might be exposed as well. And thirdhand smoke is difficult to【C17】________. It’s virtually impossible to remove this stuff【C18】________you remove the flooring and wall.
    Experts say much more work needs to be done on the【C19】________of the problem, the health risks, and effective ways to clean up the compounds. They advise parents not to expose their children to thirdhand smoke, not to【C20】________hotel rooms or cars used by smokers.
【C18】
选项 A、unless
B、while
C、if
D、once
答案A
解析 三手烟存在于各种地方，可见只有打掉地板和墙(remove the flooring and wall)，才能清除三手烟。A项unless“除非”满足语义和逻辑要求。B项while“当……”表时间。C项if“如果”表因果。D项once“一旦”表条件。三项代入后均不符合句子逻辑。
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