A、It’s useless. B、It’s unnecessary. C、It’s doubtful. D、It’s helpful. D

Since 1939, numerous scientific studies have been conducted to determine whether smoking is a health hazard. The trend of the evidence has been consistent and indicates that there is a serious health risk Research teams have conducted studies that show beyond all reasonable doubt that cigarette smoking is associated with a shortened life expectancy.
    Cigarette smoking is believed by most research workers in this field to be an important factor in the development of cancer of the lungs and cancer of throat. Male cigarette smokers have a higher death rate from heart disease than nonsmoking makes. Female smokers are thought to be less affected because they do not breathe in the smoke so deeply.
    It might be helpful to look at what smoking actually does to the human body. Smoke is a mixture of gases, vaporized chemicals, minute particles of ash, and other things. There is also nicotine, which is a powerful poison and black tar. As the smoke is breathed in, all these components form deposits on the membranes of the lungs. One point of concentration of deposits is where the air tube divides. Most lung cancer begins at this point.
    Smoking also affects the heart and Blood vessels. It is known to be related to Beurger’ s disease, a narrowing of the small veins in the hands and feet that can cause great pain. Smokers also die much more often from heart disease.