There is evidence that the usual variety of high blood pressure is, in part, a familial disease. Since families have similar genes as well as similar environments, familial diseases could be due to shared genetic influences, to shared environmental factors, or to both. For some years, the role of one
　　environment factor commonly shared by families, namely dietary salt (i.e., sodium chloride), has been studied at Brookhaven National Laboratory. These studies suggest that long excess salt intake 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 developed 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 long successive generations of those animals that failed to develop hypertension from salt intake, 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, sensitive strain (the "S" strain) has also been developed.
　　The availability of these two strains permits investigations possible. They provide a plausible laboratory model on which to investigate some clinical aspects of the human hypertension. More important, there might be the possibility of developing methods by which genetic susceptibility (敏感性) of human beings to high blood pressure can be defined without waiting for its appearance. Radioactive sodium 22 was an important "tool" in working out the characteristics of the sodium chloride metabolism.
　　21. The study of the effects of salt on high blood pressure was carried out_______.
　　A. as 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 proven that salt caused high blood pressure
　　D. because of the availability of chemically pure salt and its derivatives
　　22. The main difference between "S" and "R" rats is their_______.
　　A. need for sodium 22 B. rate of mating
　　C. reaction to salt D. type of blood
　　23. We can infer from the article that sodium 22 can be used to_______.
　　A. control high blood pressure
　　B. cure high blood pressure caused by salt
　　C. tell the "S" rats from the "R" rats
　　D. determine what a sodium chloride metabolism is like
　　24. The most beneficial results of the research might be_______.
　　A. development of diets free of salt
　　B. an early cure for high blood pressure
　　C. control of genetic agents that cause high blood pressure
　　D. the early identification of potential high blood pressure victims
　　25. Which of the statements best relates the main idea of this article?
　　A. When salt is added rats and human beings react similarly.
　　B. The near future will see a cure for high blood pressure.
　　C. The medical field is desperately in need of research.
　　D. A tendency toward high blood pressure may be a hereditary factor.
　　21. A 22. C 23. D 24. D 25. D

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