题目内容
Sunbath and Skin Cancer
In the summer, millions of Americans will offer up their bodies to the sun rays. A tan indicates health and beauty, and most sun worshippers will sacrifice a lot to achieve it, including themselves.
With each hour, the sun's ultraviolet (紫外线的) radiation produces irreversible (不可逆的) damage, hastening the development of unsightly wrinkles. And with each year on the beach or rooftop, the sunbather increases his risk of getting skin cancer. Skin cancer is by far the most common form. of cancer. An estimated 400,000 new cases will be detected this year in the United States, and almost all of them can be blamed on over-exposure to the sun.
Fortunately ,most of these cancers are highly curable. But they can be disfiguring and take time to treat. For that reason, sun worshippers should treat deity with a good deal of awe(敬畏). Sunburn, of course, is the initial hazard posed by UV radiation. Prolonged exposure to UV, however, interferes with the production of collagen fibers in the dermis (真皮层), causing the skin to lose elasticity (弹性) and creating premature wrinkles. Further deterioration (恶化) of the dermis deprives the epidermis (表皮层)of nutrition and causes it to become thin and dry.
Cancer is UV's final result. Shortwave radiant energy, especially from the UV-B band, breaks the strands of DNA.Enzymes (酶) work constantly to rearrange the (变异的) DNA into proper sequence, but with repeated UV exposure, the repair process may eventually break down. Then the mutant DNA may produce a colony of cancer cells. But skin cancer may be avoided with a good dose of common sense. People with fair skin and blue eyes who burn easily stand the highest risk. Special danger spots are the parts of the body most constantly exposed to the sun, such as the cheeks, nose, lower lip and the ears. People who have already developed precancerous (癌变前的) lesions (伤害) or had one skin-cancer growth stand a greatly increased chance of developing others.
Dermatologists (皮肤科医生) recommend avoiding the sun when it is most intense between 11 a.m. and 3 p. m. Anyone who insists on sunbathing should use a good sun screen. These lotions and salves contain chemicals that block out the burning UV-B radiation while permitting the tanning rays to reach the skin.
Which of the following is the result, if a sunbather stays on the beach for a long time？
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【H4】
【H6】
During its growth, a large tree takes in with the help of its roots many tons of water. Yet the tree keeps only very little of the water, perhaps one percent. The rest is given off through its leaves into the air.
When we think of a whole forest with so many trees, the total must clearly be very great, even though the amounts may be different according to the kind of tree.
A tree can be seen as a kind of pump (水泵) in two ways. It is always sucking up water from the ground, which would otherwise become far more loosely made up. At the same time, it increases the humidity(湿度) of the surrounding land, which would otherwise be much drier.
Trees and forests also act as a breakwater(防水堤) against rainstorms, which, especially on hillsides, would otherwise wash away much of the topsoil, both around the trees and lower down. The rich humus(腐殖土) that forms the floor of the forests acts as a sponge(海绵). It holds far more water than the soil with little humus.
A tree sucks up water through its ______.
【H9】
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