单选题 {{B}}Passage Four{{/B}}

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.

单选题 The study of the effects of salt on high blood pressure was carried out ______.

【正确答案】 A

【答案解析】[解析] 推断题。第一段第一、二句告诉我们一般的高血压病与家族病史有关，成因可能是某家族共有的基因或环境因素，或是二者的综合。第三句进一步指出食盐就是构成家庭成员共同的环境因素之一。由此可知，这一因素可能对高血压的形成等有着某种关联。A比较符合原文的意思。选项B描述的是研究这一活动，选项C描述的是研究的结论，都不是其原因；原文未提到“chemically pure salt(化学意义上的纯盐)”，这一物质是展开该研究的条件，故D也错。

单选题 It can be implied that the main difference between "S" and "R" strains is in their ______.

【正确答案】 C

【答案解析】[解析] 推断题。原文末段“Radioactive (放射性的) sodium 22 was an important 'tool' in working out the characteristics of the sodium chloride metabolism.”表明sodium 22是用来研究食盐特性的“工具”，不能区分“R”型与“S”型动物，故A错；第二段指出这两组动物都摄入盐，但未提摄入量的大小，故B错：D项脱离了原文内容。请注意，涉及原文特定表述语时切忌脱离上下文臆测其含义，而应联系原文内容确定其具体含义。

单选题 It is implied in the second paragraph that ______.

【正确答案】 D

【答案解析】[解析] 推断题。原文第二段描述实验动物分组时使用了only一词，“By mating in successive generations only those animals that... In contrast, by mating only animals that...”，强调同组动物的基因纯粹性，当导致高血压的因素限定为环境因素即盐的摄取时，不同组动物各自基因与高血压间的关联便显现出来了。也就是说对盐反应不同类型的动物交配的后代将无法体现盐与高血压之间的关系。D项“高血压可能受交配冈索的影响”是最合乎逻辑的推论了。

单选题 Among the results of the research discussed in this passage, the most beneficial might be ______.

【正确答案】 B

【答案解析】[解析] 推断题。原文第三段指出“More important, ... without waiting for its appearance.”即可能有办法能够将那些具有高血压基因的人在他们患该病之前就鉴别出来。B项“对可能的高血压患者进行早期甄别”应该是最相吻合的答案了。原文未提及A、D两项；原文强调的是未患病前将可能患该病的人找出来，不是说人得病后某种早期治疗方法，故C错。