阅读理解
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 mean 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.
【答案解析】根据文中第一段的“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，判断人的视力是否完好的标准是在看斯内伦视力表的时候是否能毫无障碍地达到20／20的视力。测试值取决于从20英尺之外，你能看到多少行大小不一的字母。据此可知，A项正确。
单选题27.How is hawk's eyesight better than ours?
【正确答案】
B
【答案解析】根据文中第一段的“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)”可知，鹰的视力非常好，当它盘踞在帝国大厦的楼顶时，它能看清在人行道上的一角硬币。据此可知，鹰眼可以从更远的距离看清物体，这是鹰眼强于人眼的地方，B项正确。
单选题28.Monocular vision enables the kingfisher to______.
【正确答案】
C
【答案解析】根据文中第一段的“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．”可知，其中一个凹窝让飞翔在空中的翠鸟每次用一只眼睛扫视水下的情况，这被称为单眼视觉。据此可知，单眼视觉让翠鸟能够一次转动一个眼睛，C项正确。
单选题29.A bee finds its direction by______.
【正确答案】
C
【答案解析】根据文中第二段的“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．”可知，有了这种视觉，蜜蜂只把太阳看作一个单点，一个不变的参照点。所以，它的眼睛是一个极好的导航仪器，能不断地测量其飞行线路与太阳的角度。据此可知，蜜蜂是通过太阳这个恒定的参照点来辨别方向的，C项正确。