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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

admin2019-06-23  86
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 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.
How is hawk’s eyesight better than ours?
选项 A、It can identify small items more quickly than we do.
B、It can see at a longer distance than we do.
C、It has more cones in the retina than we do.
D、It has bigger eyes than we do.
答案B
解析 首段第5句中的while perched on top of the Empire State Building表明本句强调的是鹰能看到距离很远的物件，由此可见，选项B为本题答案。从首段第5句中的dime一词可以推断鹰可以看到很小的物件，但原文并没有暗示它辨认物件的速度，更没有在这方面与人眼作比较，因此选项A不正确；首段第6句只是指出鹰眼的视网膜每平方毫米有100万个圆锥细胞，但并没有对比人眼和鹰眼所具有的圆锥细胞的总数，因此选项C不正确；原文并未提及与选项D有关的内容。
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