Questions 6 to 8 are based on the following readingpassage.
The success of fluoride in combating dental decay is well established and, without a doubt, sociall beneficial.However, fluoride's toxic properties have been known fora 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.
6. The passage suggests that it would be easier to calculate fluoride intake from food if
○ A adequate diets were available for most people.
○ B individual eating habits were more uniform
○ C the fluoride content of food was more varied
○ D more people were aware of the fluoride content of food
○ E methods for measuring the fluoride content of food were more generally agreed on
7. One function of the second paragraph of the passage is to
○ A raise doubts about fluoride's toxicity
○ B introduce the issue of fluoride's toxicity
○ C differentiate a toxic from a nontoxic amount of fluoride
○ D indicate that necessary knowledge of fluoride remains incomplete
○ E discuss the foodstuffs that are most likely to contain significant concentrations of fluoride
For the following question, consider each of the choices separately and select all that apply
8.The passage suggests which of the following about the effect of fluoride on humans?
□ A The effect of fluoride intake from water and air is relatively difficult to monitor.
□ B An intake of 4 milligrams over a long period of time usually leads to a skeletal disorder in humans.
□ C An intake of slightly more than 4 milligrams for only a few months is not likely to be life-threatening.
正确答案：6.B 7. D 8. C
