单项选择题

Passage Four

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 environmental factor commonly shared by families, namely dietary salt (i.e. sodium chloride), has been studies at Brook Haven National Laboratory. These studies suggest that frequent excess salt eating 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 high blood pressure whereas a few rapidly developed very serious high blood pressure 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 high blood pressure from salt eating, a resistant strain (the "R" strain) has been evolved in which consumption of large quantities of salt falls to influence the blood pressure significantly, in contrast, by mating only animals that quickly develop high blood pressure from salt, a sensitive strain ("S" strain) has also been developed. The availability of these two strains permits investigations not possible formerly. They provide a likely laboratory model on which to investigate some clinical (临床的) aspects of the human types of high blood pressure. More important, there might be the possibility of developing methods by which genetic likelihood of human beings to develop high blood pressure can be defined without waiting for its appearance. Radioactive (放射性的) sodium 22 was an important "tool" in working out the characteristics of the sodium chloride metabolism.

We can infer from the passage that ______.