OG15 RC80求助

芊层酥

文章是这样的： It was once believed that the brain was independent of metabolic processes occurring elsewhere in thebody. In recent studies, however, we have discovered that the production and release in brain neurons ofthe neurotransmitter serotonin (neurotransmitters are compounds that neurons use to transmit signals toother cells) depend directly on the food that the body processes.


Our first studies sought to determine whether the increase in serotonin observed in rats given a largeinjection of the amino acid tryptophan might also occur after rats ate meals that change tryptophan levelsin the blood. We found that, immediately after the rats began to eat, parallel elevations occurred in bloodtryptophan, brain tryptophan, and brain serotonin levels. These findings suggested that the production andrelease of serotonin in brain neurons were normally coupled with blood-tryptophan increases. In laterstudies we found that injecting insulin into a rat’s bloodstream also caused parallel elevations in blood andbrain tryptophan levels and in serotonin levels. We then decided to see whether the secretion of the animal’sown insulin similarly affected serotonin production. We gave the rats a carbohydrate-containing meal thatwe knew would elicit insulin secretion. As we had hypothesized, the blood tryptophan level and theconcentrations of tryptophan and of serotonin in the brain increased after the meal.


Surprisingly, however, when we added a large amount of protein to the meal, brain tryptophan andserotonin levels fell. Since protein contains tryptophan, why should it depress brain tryptophan levels? Theanswer lies in the mechanism that provides blood tryptophan to the brain cells. This same mechanism alsoprovides the brain cells with other amino acids found in protein, such as tyrosine and leucine. Theconsumption of protein increases blood concentration of the other amino acids much more,proportionately, than it does that of tryptophan. The more protein is in a meal, the lower is the ratio of theresulting blood-tryptophan concentration to the concentration of competing amino acids, and the moreslowly is tryptophan provided to the brain. Thus the more protein in a meal, the less serotonin subsequentlyproduced and released.


问：The authors’ discussion of the “mechanism that provides blood tryptophan to the brain cells” （第三段） is meant to
(C) help explain why a particular research finding was obtained
(D) provide supporting evidence for a controversial scientific theory

答案是C。可是从全文看，第三段只是提出了另一个研究结果，最终是用来支持“production and release in brain neurons depend on food”这个中心论点的啊。答案C会不会片面了一点？
OG说文章并没有theory, 但中心论点不就是一个theory么？
这种类似的结构题（问到某句话某段话的作用），到底是应该答某一段的作用呢，还是答它对全文的作用？


谢谢~

DanielGi

个人经验，供参考。答这种某个词，某个句子在文中作用的题，直接定位原文，然后知道他的上一句，看看他们之间是什么关系就可以了。

lanheer

DanielGi 发表于 2017-2-3 23:26
个人经验，供参考。答这种某个词，某个句子在文中作用的题，直接定位原文，然后知道他的上一句，看看他们之 ...

一般看上一句就好。不用看下一句么

DanielGi

lanheer 发表于 2017-2-5 21:03
一般看上一句就好。不用看下一句么

具体还没总结过，不过我发现大多数都是看上一句就行。
做一个推断性分析哈：我觉得在科学和现象描述这类文章，大多是总-分结构，所以基本上是第一段提出中心论点，第二段段首提出实验目的或方法，然后详细说明实验的做法（包括样本选取，实验流程）以及实验中各参数的关系。第三段有可能会出现反常现象（提出反例），或者通过进一步说明来证明结论的正确性。
故此，大多数题型在问到某一个例子，或句子有什么用的时候，首先应该想到的就是他的前面（这个是由写作方法决定的）是否有总领性的语句。