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Biologists have advanced two theories to explain why schooling of fish occurs in so many fish species. Because schooling is particularly widespread among species of small fish, both theories assume that schooling offers the advantage of some protection from predators.

Proponents of theory A dispute the assumption that a school of thousands of fish is highly visible. Experiments have shown that any fish can be seen, even in very clear water, only within a sphere of 200 meters in diameter. When fish are in a compact group, the spheres of visibility overlap. Thus the chance of a predator finding the school is only slightly greater than the chance of the predator finding a single fish swimming alone. Schooling is advantageous to the individual fish because a predator's chance of finding any particular fish swimming in the school is much smaller than its chance of finding at least one of the same group of fish if the fish were dispersed throughout an area.

However, critics of theory A point out that some fish form schools even in areas where predators are abundant and thus little possibility of escaping detection exists. They argue that the school continues to be of value to its members even after detection. They advocate theory B, the "confusion effect," which can be explained in two different ways.

Sometimes, proponents argue, predators simply cannot decide which fish to attack. This indecision supposedly results from a predator's preference for striking prey that is distinct from the rest of the school in appearance. In many schools the fish are almost identical in appearance, making it difficult for a predator to select one.The second explanation for the "confusion effect" has to do with the sensory confusion caused by a large number of prey moving around the predator. Even if the predator makes the decision to attack a particular fish, the movement of other prey in the school can be distracting. The predator's difficulty can be compared to that of a tennis player trying to hit a tennis ball when two are approaching simultaneously.

The author is primarily concerned with

discussing different theories analyzing different techniques defending two hypotheses refuting established beliefs revealing new evidence

正确答案： A

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文章大意逻辑
生物学家提出两种理论解释鱼类成群游动现象，都认为成群游动能抵御捕食者。理论A支持者认为鱼群不易被发现，实验表明鱼的可见范围有限，鱼群聚集使捕食者发现单条鱼概率降低。但理论A批评者指出有些鱼在捕食者多的地方也成群，提出理论B“混淆效应”，可从两方面解释。 文章类型及逻辑简图
文章类型为新旧观点对比。 Biologists: two theories to explain schooling protection from predators Theory A: dispute highly visible assumption visibility overlap less chance for predator to find individual fish Critics of Theory A: fish form schools in predator abundant areas Theory B: "confusion effect" 做题方法及问题类型
本题为主旨题，通过梳理文章整体结构，把握作者围绕两种理论展开讨论这一核心来解题。 选项分析
A. discussing different theories：文章确实在讨论关于鱼类成群游动的不同理论，符合主旨，正确。 B. analyzing different techniques：文章未提及不同技术，无中生有，错误。 C. defending two hypotheses：文章只是陈述理论，并非为两个假设辩护，错误。 D. refuting established beliefs：文章重点不是反驳已有的信念，错误。 E. revealing new evidence：新证据不是文章主要关注的，错误。

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