共用题干Vision
Human vision like that of other primates（灵长目）has evolved in an arboreal（丛林）environment.______(46)
In the course of evolution,members of the primate line have acquired large eyes while the nose has shrunk.______ (47) Of mammals（哺乳动物）only humans and some primates enjoy color vision.______(48)Horses live in a single-color world.
Light visible to human eyes,however,occupies only a very narrow band in the whole electromagnetic spectrum（光谱）.Ultraviolet rays（紫外线）are invisible to humans though ants and honeybees are sensitive to them.______(49)The world would look terribly different if human eyes were、 sensitive to infrared radiation（红外线）.Then instead of the darkness of night,we would be able to move easily in a strange shadowless world.______(50)
But human eyes excel in other ways. They are in fact remarkably discerning in color gradation. The color sensitivity of normal human vision is rarely surpassed even by complicated technical devices. _________（50）A:Humans have no direct perception of infrared rays.B:In the dense complex world of a tropical forest,it is more important to see well than to develop an acute sense of smell.C:That gives the eye a wilder view.D:The red flag is black to the bull.E:There are different mammals in tropical forest.F: In this world,objects glowed with varying degrees of intensity.