Sunlight is a great source of vitamin D in most of the world. Yet a (an)【C1】______number of experts think that many people aren’

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问题     Sunlight is a great source of vitamin D in most of the world. Yet a (an)【C1】______number of experts think that many people aren’t getting enough vitamin D—【C2】______those who work and play indoors.
    It’s long been known that vitamin D is【C3】______for strong bones. Research in the last several years has shown that low levels of vitamin D may increase the risk of heart【C4】______in men and deaths from some cancers. But new research suggests that it also protects against a wide【C5】______of diseases. Low levels of vitamin D have also been【C6】______with various forms of cancer and high blood pressure, stroke,【C7】______illness etc. The strongest source,【C8】______far, is ultraviolet (紫外线) B rays from the sun, which【C9】______a form of cholesterol (胆固醇) into vitamin D in the skin. But UVB rays【C10】______greatly depending on cloud cover, time of year and time of day. Dark-skinned and elderly people don’t【C11】______vitamin D from the sun as【C12】______as younger, fair-skinned people. UVB rays also don’t【C13】______glass or sunscreen with a factor of 8 or more. It’s difficult to get much vitamin D through diet. Few foods【C14】______it naturally. It’s widely【C15】______that most people need some supplemental vitamin D—the question is, how much? Current U. S. guidelines, issued in 1997,【C16】______for 200 international units from birth through age 50. The National Osteoporosis Foundation【C17】______adults over age 50 to get at least 800 to 1,000 IUs to【C18】______against fractures (骨折).
    But as some experts research carry【C19】______the experiment, they worry that if people take too much vitamin D, it might act as a poison. Also, skin doctors warn people to be careful with sun【C20】______because of the risk of skin cancer.
【C12】

选项 A、exceedingly
B、excessively
C、extraordinarily
D、efficiently

答案D

解析 副词辨义题。根据常识可知,老年人借助日光合成维生素D的能力肯定弱于年轻人,故本句意为“皮肤黝黑者和老年人借助日光合成维生素D的能力弱于年轻人和肤色较浅的人”,efficiently“有效地,效率高地”符合题意。exceedingly“非常,极其”;excessively“过分地,极度”;extraordinarily“非常,格外,非凡地”,均不符合文意,故排除。
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