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  3. The human criterion for perfect vision is 20/20 for reading the standard lines on a Snellen eye chart without a hitch. The score

The human criterion for perfect vision is 20/20 for reading the standard lines on a Snellen eye chart without a hitch. The score

admin2010-11-02  40
问题      The human criterion for perfect vision is 20/20 for reading the standard lines on a Snellen eye chart without a hitch. The score is determined by how well you read lines of letters of different sizes from 20 feet away. But being able to read the bottom line on the eye chart does not approximate perfection as far as other species are concerned. Most birds would consider us very visually handicapped. The hawk, for instance, has such sharp eyes that it can spot a dime on the sidewalk while perched on top of the Empire State Building. It can make fine visual distinctions because it is blessed with one million cones (视锥细胞) per square millimeter in its retina (视网膜). And in water, humans are farsighted, while the kingfisher, swooping down to spear fish, can see well in both the air and water because it is endowed with two foveae(凹窝)-areas of the eye, consisting mostly of cones, that provide visual distinctions. One fovea permits the bird, while in the air, to scan the water below with one eye at a time. This is called monocular vision. Once it hits the water, the other fovea joins in, allowing the kingfisher to focus both eyes, like binoculars, on its prey at the same time. A frog’s vision is distinguished by its ability to perceive things as a constant motion picture. Known as "bug detectors", a highly developed set of cells in a frog’s eyes responds mainly to moving objects. So, it is said that a frog sitting in a field of dead bugs wouldn’t see them as food and would starve.
     The bee has a "compound" eye, which is used for navigation. It has 15,000 facets that divide what it sees into a pattern of dots, or mosaic. With this kind of vision, the bee sees the sun only as a single dot, a constant point of reference. Thus, the eye is a superb navigational instrument that constantly measures the angle of its line of flight in relation to the sun. A bee’s eye also gauges flight speed. And if that is not enough to leave our 20/20 "perfect vision" paling into insignificance, the bee is capable of seeing something we can’t ultraviolet light. Thus, what humans consider to be "perfect vision" is in fact rather limited when we look at other species.
However, there is still much to be said for the human eye. Of all the mammals, only humans and some primates can enjoy the pleasures of color vision.  
What is the main idea of the passage?
选项 A、The differences between human eyes and other species’ eyes.
B、The advantages of other species’ eyes in comparison with human eyes.
C、The factors that make other species’ eyes better than human eyes.
D、The standard that determines the "perfect vision" of human eyes.
答案A
解析 首段前3句通过描述人类测试视力的方法引出人类的视力不如其他物种这个话题，然后作者举了4个例子说明这个观点，而在第2段末句作者又指出人类视力在某一方面优于其他物种，可见本文主要通过举例说明人类和其他物种在视力上的不同，A概括了全文内容。
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