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.

According to the passage, both theory A and theory B have been developed to explain how

 A fish hide from predators by forming schools

 B forming schools functions to protect fish from predators

 C schooling among fish differs from other protective behaviors

 D small fish are able to make rapid decisions

 E small fish are able to survive in an environment densely populated by large predators

【申友解析】

正确答案：B

问理论A和B的共同目的。第二段是解释理论A，第四段是解释理论B，第三段是两者的区别，都不是定位点。只有第一段里有相关信息。第一段最后一句话说两者都认为鱼群对鱼的个体提供保护，躲避捕食。5个选项中只有B选项符合要求。

A选项说藏起来不让捕食者看见，只有理论A有相关内容，理论B没有；

B选项与原文之间就是原义改写的关系；

C选项说鱼的群居与其他保护措施不一样，文中未提及；

D选项说小鱼做决定快，亦属于无中生有的内容；

E选项小鱼容易在捕食者密集的地方存活，文章依然没有提到

因此，正确答案为B