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, theory B states that which of the following is a factor that enables a schooling fish to escape predators?

 A the tendency of fish to form compact groups

 B the movement of other fish within the school

 C the inability of predators to detect schools

 D the ability of fish to hide behind one another in a school

 E the great speed with which a school can disperse

【申友解析】

正确答案：B

这题问理论B说哪个因素会帮助鱼群摆脱捕食者，定位点很明确，是“理论B”底下的细节，定位到最后一段。能回答问题的一共有两点，第一点在“Sometimes, proponents argue”到“to select one”之间，说鱼数量多导致捕食者难以决定究竟吃谁；第二点在“The second explanation”到文章结尾，说鱼群中大量鱼的运动会使捕食者分心而难以捕食成功。所以这两点因素（数量多和鱼群运动）都能够正确回答这个问题。

A选项鱼群形成牢固的整体，文章中从未提及，属于无中生有的内容；

B选项说鱼群中鱼的运动，与原文中第二点相符；

C选项说捕食者侦测不到鱼群，是文章中理论A里面的内容，提及但无关；

D选项说鱼躲在其他鱼的身后，又是文中没有提到的内容；

E选项说鱼群能高速分散，文章没有提及这一点

因此，正确答案为B