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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

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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 perching on top of the Empire State Building. It can make fine visual distinctions because it is blessed with one million cones 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.
The Snellen eye chart measures one’s eyesight by______.
选项 A、the number of lines he reads
B、the distance he stands away from the chart
C、the speed at which he recognizes the letters
D、several integrative factors
答案D
解析 根据第一段中的“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．”可知，判断一个人视力是否完好的标准是看他在看斯内伦视力表时是否能毫无障碍地达到20／20的视力。测试值取决于在20英尺之外，一个人能看到多少行大小不一的字母。据此可知，D项正确。
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