Only three strategies are available for controlling cancer: prevention, screening and treatment. Lung cancer causes more deaths

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问题      Only three strategies are available for controlling cancer: prevention, screening and treatment. Lung cancer causes more deaths than any other types of cancer. A major cause of the disease is not  【21】______  known; there is no  【22】______  evidence that screening is much help; and treatment  【23】______  in about 90 percent of all cases. At present, therefore, the main strategy must be  【24】______  . This may not always be true, of course, as for some other types of cancer, research  【25】______  the past few decades has suggested some importance in prevention, screening or treatment.  【26】______  , however, we consider not what research may one day offer but what today’s knowledge could already deliver that is not being delivered, then the most practicable and cost-effective opportunities for  【27】______  premature death from cancer, especially lung cancer, probably involve neither screening nor improved treatment,  【28】______  prevention.
     This conclusion does not depend on the unrealistic assumption that we can  【29】______  tobacco. It merely assumes that we can reduce cigarette sales appreciably by raising prices or by  【30】______  on the type of education that already appears to have a  【31】______  effect  【32】______  cigarette assumption by white-collar workers and that we can substantially reduce the amount of tar  【33】______  per cigarette.
     The practicability of preventing cancer by such measures applies not only in those countries, such as, the United States of America, because cigarette smoking has been common for decades, 25 to 30 percent of all cancer deaths now involves lung cancer, but also in those where it has become  【34】______  only recently. China is a good example. Countries where cigarette smoking is only becoming widespread can expect enormous increase in lung cancer during the 1990’s or early in the next century,  【35】______  prompt effective action is taken  【36】______  the habit.
     There are three reasons  【37】______  the prevention of lung cancer is of such overwhelming importance: first, the disease is extremely common, causing more deaths than any other type of cancer now  【38】______  ; secondly, it is generally incurable; and finally,  【39】______  tobacco consumption will also have a substantial  【40】______  on many other diseases.
【35】

选项 A、though
B、when
C、unless
D、if

答案C

解析 unless意为“除非,如果不”。本句意为:如果对于吸烟这一习惯不及时采取有效措施,患肺癌的人数会有很大的增加。故应选C。
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