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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

admin2017-02-01  71
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.
A bee finds its direction by _____.
选项 A、dividing what it sees into thousands of dots
B、constantly gauging its flying speed
C、using the sun as a constant point of reference
D、measuring the angle of the sun
答案C
解析 第2段第3、4句描述了蜜蜂利用太阳导航的机制，将选项与这些细节对照可以知道选项C为正确的说法。选项A是蜜蜂眼睛的成像机制，虽然眼睛必然与导航有关，但单靠成像并不能起到导航的作用，因此选项A不是蜜蜂寻找方向的方法；选项B中的gauging its flying speed也跟蜜蜂寻找方向无关；第2段第4句表明蜜蜂测量的是飞行路线和太阳的夹角，而不是测量太阳的角度，因此选项D不正确。
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