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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 establi

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

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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。
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