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  3. There is evidence that the usual variety of high blood pressure is, in part, a family disease. Since families have similar genes

There is evidence that the usual variety of high blood pressure is, in part, a family disease. Since families have similar genes

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问题     There is evidence that the usual variety of high blood pressure is, in part, a family disease. Since families have similar genes as well as similar environment, family diseases could be due to shared genetic influences, to shared environmental factors, or to both. For some years, the role of one’s environmental factor commonly shared by families, namely, dietary salt, has been studied at Brookhaven National Laboratory. These studies suggest that chronic excess salt ingestion can lead to high blood pressure in man and animals. Some individuals, however, and some rats consume large amounts of salt without developing high blood pressure. No matter how strictly all environmental factors were controlled in these experiments, some salt - fed animals never develop hypertension whereas  a few rapidly developed very severe hypertension followed by early death. These marked variations were interpreted to result from differences in genetic constitution.
    By mating in successive generations only those animals that failed to develop hypertension from salt ingestion, a resistant strain (the" R" strain) has been evolved in which consumption of large quantities of salt fails to influence the blood pressure significantly. In contrast, by mating only animals  that quickly develop hypertension from salt, a sensitive strain ("S" strain) has also been developed.
    The availability of these two strains permits investigations not heretofore possible. They provide a plausible laboratory model on which to investigate some clinical aspects of the human proto - types of hypertension. More important, there might be the possibility of developing methods by which genetic susceptibility of human beings to high blood pressure can be defined without waiting for its appearance.
The study of the effects of salt on high blood pressure was carried out ______.
选项 A、because members of the same family tend to use similar amounts of salt
B、to explore the long - term use of a sodium - based substance
C、because it was proved that salt caused high blood pressure
D、because of the availability of chemically pure salt and its derivatives
答案A
解析 “salt"是环境因素之一，在确定family这一点以后，研究salt才可看出结果，所以答案为A
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