托福阅读中的背景知识介绍及解题技巧

【Part1. 背景知识】

什么是浮游植物？

Phytoplankton, a flora of freely floating, often minute organisms that drift with water currents. Like land vegetation, phytoplankton uses carbon dioxide, releases oxygen, and converts minerals to a form animals can use. In fresh water, large numbers of green algae often colour lakes and ponds, and cyanobacteria（蓝细菌） may affect the taste of drinking water.

浮游植物是一种自由漂浮的植物，通常很小，能够随水浪漂浮。如果陆地上的植被，浮游植物吸收二氧化碳，释放氧气，能够将矿物质转化成植物可以利用的形式。在淡水中，大量的绿色藻类可以改变湖泊或者池塘的颜色，蓝细菌可以影响饮水的味道。

我们日常生活中见到的大多数藻类都是属于浮游植物，如硅藻等。

浮游植物的特征

Oceanic phytoplankton is the primary food source, directly or indirectly, of nearly all sea organisms.

几乎所有的海洋生物都直接或者间接以海洋里的浮游植物为主要的食物来源。

Phytoplankton varies seasonally in amount, increasing in spring and fall with favourable light, temperature, and minerals.

浮游植物的数量会季节性变化，一般会在春秋增加，因为提供了有利的光线，温度和矿物质。

TPO35-1文章大意

TPO文章中会涉及到大量的学科知识，在我们学习备考之时，了解学科背景，也是非常重要的一个环节，而TPO阅读文章本身其实就能给我们很多背景介绍，通过刷题之后总结段落大意也是提升背景知识的一个好方法。

在TPO35-1这篇文章中，最开始提出浮游植物具有季节性更替的现象。在随后的段落中，对这一现象进行了诸多可能的解释，最开始提出温度可能是季节更替的原因之一，但并不是原因。然后又提出海水中所含的营养水平变化也可能是季节更替的原因，但也不是原因。随后给出第三个可能的解释：海水原有的浮游植物的新陈代谢可能是更替的原因。在新陈代谢这一解释里文章进行了具体的展开，一方面新陈代谢能产生有毒物质，另一方面新陈代谢能产生维生素。尽管新陈代谢对更替很重要，但是其真正作用还需要更多证据。之后文章又给出第四个解释：食草动物也可能是浮游生物更替的原因。在文章结束处，作者表示所有的上述因素都在同时起作用，导致季节性更替。

【Part2 技巧解析】

细节题——选项定位法

细节题作为托福阅读中占比的题型之一，其重要性不言而喻。该题型难度系数不算太高，属于考试中各分数段学员都要努力做到必得分的一个题型。

但在当前的托福阅读考试中，不论是正向细节题还是反向选错的except类的细节题，题干无法提取关键词定位或者关键词分布在整个段落的情况逐渐趋多，在这种情况下，传统的题干提取关键词，带着关键词回文章定位的方法变得不再适用，更好的方法是从选项中提取关键词进行定位，今天我们通过TPO35-1的两道题来讲解一下选项定位法。

Paragraph 4: Yet another explanation is that species succession is a consequence of changes in seawater brought about by the phytoplankton living in it. Each species of phytoplankton secretes or excretes organic molecules into the seawater. These metabolites can have an effect on the organisms living in the seawater, either inhibiting or promoting their growth. For any individual organism, the amount of metabolite secreted is small. But the effect of secretions by all the individuals of the dominant species can be significant both for themselves and for other species.

4. According to paragraph 4, all of the following are true about metabolites secreted by phytoplankton EXCEPT

A.They have a less significant effect on the dominant species of phytoplankton than they do on other species of phytoplankton.

B.They affect the growth of phytoplankton living in seawater.

C.They are secreted in small amounts by individual phytoplankton.

D.They cause changes in seawater that result in species succession of phytoplankton.

正确答案：A

此题根据题干关键词phytoplankton定位，但phytoplankton是文章标题词，并不能给有效缩小阅读范围，所以更好的方法是用选项中的关键词回文章定位的选项定位法。

A选项中选取dominant species作为定位词，可以定位到文章中最后一句But the effect of secretions by all the individuals of the dominant species can be significant both for themselves and for other species. 文章说对dominant species和别的species的重要性是相同的，而选项A说对dominant species的影响更小，信息错误，在except题中，此信息错误选项应该为正确答案。

B选项提取growth作为关键词回文章定位，对应文章：These metabolites can have an effect on the organisms living in the seawater, either inhibiting or promoting their growth. 与B选项表达一致，不选。

C选项用small amounts作为定位词，对应文章：For any individual organism, the amount of metabolite secreted is small. 与C选项表达一致，不选。

D选项用species succession作为定位词，对应文章：Yet another explanation is that species succession is a consequence of changes in seawater brought about by the phytoplankton living in it. 与D选项表达一致，不选。

通过这一题，我们来系统性的总结一下，选项定位的方法应该如何使用。

解题技巧：

步：审题, 提取问题主干词, 定位难度大

情况1：主干词为文章标题词

情况2：主干词为段落主题词

情况3：主干部分的词为四级基础词, 在原文容易被改写, 定位难度大

第二步：通读选项, 根据选项的名词回原文定位（优先找有专有名词的选项）, 对照选项和原文说法是否一致。

第三步：验证选项是否符合题意

接下来，我们带着上面总结的技巧，来完成一下TPO35-1的第5题。

Paragraph 5: These organic metabolites could, and probably do, include a number of different classes of organic compounds. Some are likely toxins, such as those released by the dinoflagellates (a species of plankton) during red tides, which inhibit growth of other photosynthetic organisms. In such cases, the population explosion of dinoflagellates is so great that the water becomes brownish red in color from the billions of dinoflagellate cells. Although each cell secretes a minute amount of toxin, the massive dinoflagellate numbers cause the toxin to reach concentrations that kill many creatures. This toxin can be concentrated in such filter-feeding organisms as clams and mussels, rendering them toxic to humans.

5.According to paragraph 5, all of the following statements are true about dinoflagellates EXCEPT

A.They produce toxins that inhibit the growth of other organisms.

B.Each produces a very little amount of toxin.

C.Their toxins can concentrate in clams and mussels.

D.Their toxins are dangerous only to other photosynthetic organisms.

正确答案：D

步：审题——题干dinoflagellates为段落中心词，所以可以确定本题选取题干定位法。

第二步：通读选项, 根据选项的名词回原文定位

A选项通过growth of other organisms定位

B选项定位词：a very little amount of toxin

C选项定位词：clams and mussels

D选项定位词：dangerous only to other photosynthetic organisms.

第三步：验证选项是否符合题意

A定位句：Some are likely toxins, such as those released by the dinoflagellates (a species of plankton) during red tides, which inhibit growth of other photosynthetic organisms. 与选项表达一致，排除。

B定位句：Although each cell secretes a minute amount of toxin, the massive dinoflagellate numbers cause the toxin to reach concentrations that kill many creatures. 与选项表达一致，排除。

C定位句：This toxin can be concentrated in such filter-feeding organisms as clams and mussels, rendering them toxic to humans. 与选项表达一致，排除。

D 定位句：Although each cell secretes a minute amount of toxin, the massive dinoflagellate numbers cause the toxin to reach concentrations that kill many creatures. dangerous改写成了kill many creatures，但选项说的是only to other photosynthetic organisms.原文并没有对应信息，所以信息错误，为本题except题的正确答案。

希望通过对TPO35-1这两题中选项定位方法的讲解与总结，帮助各位考生熟练掌握这一方法，在对应的情况下，熟练使用这一方法高效选择出正确答案。