The success of fluoride in combating dental decay is well established and, without a doubt (without a doubt: adv.无疑地), socially beneficial. However, fluoride’s toxic properties have been known for a century. In humans excessive intake (for adults, over 4 milligrams per day) over many years can lead to skeletal fluorosis, a well-defined skeletal disorder, and in some plant species, fluoride is more toxic than ozone, sulfur dioxide, or pesticides.
Some important questions remain. For example, the precise lower limit at which the fluoride content of bone becomes toxic is still undetermined. And while fluoride intake from water and air can be evaluated relatively easily, it is much harder to estimate how much a given population ingests from foodstuffs because of the wide variations in individual eating habits and in fluoride concentrations in foodstuffs. These difficulties suggest that we should by wary of indiscriminately using fluoride, even in the form of fluoride-containing dental products.
The passage suggests which of the following about the effect of fluoride on humans?
(A) The effect is more easily measured than is the effect of exposure to pesticides.
(B) The effect of fluoride intake from water and air is relatively difficult to monitor.
(C) In general the effect is not likely to be as harmful as the effect of exposure to sulfur dioxide.
(D) An intake of 4 milligrams over a long period of time usually leads to a skeletal disorder in humans.（E）
(E) An intake of slightly more than 4 milligrams for only a few months is not likely to be life-threatening.
我不明白为什么不选D，而选E， 感谢大家的高见！！！！！
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发表于 2009-9-15 05:11:00
